diff --git a/doc/telescope-frecency.txt b/doc/telescope-frecency.txt
index 0d0008d..671edb5 100644
--- a/doc/telescope-frecency.txt
+++ b/doc/telescope-frecency.txt
@@ -326,14 +326,15 @@ This will be called by |telescope.nvim| for its initialization. You can also
 call this to initialize this plugin separated from |telescope.nvim|'s
 initialization phase.
 
-This is useful when you want to load |telescope.nvim| lazily, but want to
+This is useful when you want to load |telescope.nvim| lazily and want to
 register opened files as soon as Neovim has started. Example configuration for
 |lazy.nvim| is below.
 >lua
     {
       "nvim-telescope/telescope-frecency.nvim",
       main = "frecency",
-      ---@type FrecencyOpts
+      -- `opts` property calls the plugin's setup() function.
+      -- In this case, this calls frecency.setup().
       opts = {
 	  db_safe_mode = false,
       },
@@ -358,7 +359,10 @@ register opened files as soon as Neovim has started. Example configuration for
 	telescope.load_extension "frecency"
       end,
     },
-
+<
+This function does nothing when it is called for the second times and later.
+If you want to set another configuration, use
+|telescope-frecency-configuration-config.setup()|.
 
 ==============================================================================
 CONFIGURATION				      *telescope-frecency-configuration*
diff --git a/lua/frecency/config.lua b/lua/frecency/config.lua
index 22bd729..e6e41d9 100644
--- a/lua/frecency/config.lua
+++ b/lua/frecency/config.lua
@@ -26,6 +26,7 @@ local os_util = require "frecency.os_util"
 
 ---@class FrecencyConfig: FrecencyRawConfig
 ---@field ext_config FrecencyRawConfig
+---@field private cached_ignore_regexes? string[]
 ---@field private values FrecencyRawConfig
 local Config = {}
 
@@ -79,6 +80,7 @@ Config.new = function()
     workspaces = true,
   }
   return setmetatable({
+    cached_ignore_regexes = {},
     ext_config = {},
     values = Config.default_values,
   }, {
@@ -138,6 +140,17 @@ Config.get = function()
   return config.values
 end
 
+---@return string[]
+Config.ignore_regexes = function()
+  if not config.cached_ignore_regexes then
+    config.cached_ignore_regexes = vim.tbl_map(function(pattern)
+      local regex = vim.pesc(pattern):gsub("%%%*", ".*"):gsub("%%%?", ".")
+      return "^" .. regex .. "$"
+    end, config.ignore_patterns)
+  end
+  return config.cached_ignore_regexes
+end
+
 ---@param ext_config any
 ---@return nil
 Config.setup = function(ext_config)
@@ -174,6 +187,7 @@ Config.setup = function(ext_config)
     workspace_scan_cmd = { opts.workspace_scan_cmd, { "s", "t" }, true },
     workspaces = { opts.workspaces, "t" },
   }
+  config.cached_ignore_regexes = nil
   config.ext_config = ext_config
   config.values = opts
 end
diff --git a/lua/frecency/database.lua b/lua/frecency/database.lua
index 36eca5a..569e25d 100644
--- a/lua/frecency/database.lua
+++ b/lua/frecency/database.lua
@@ -1,6 +1,7 @@
 local Table = require "frecency.database.table"
 local FileLock = require "frecency.file_lock"
 local config = require "frecency.config"
+local fs = require "frecency.fs"
 local watcher = require "frecency.watcher"
 local log = require "frecency.log"
 local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
@@ -13,48 +14,73 @@ local Path = require "plenary.path" --[[@as FrecencyPlenaryPath]]
 ---@field score number
 ---@field timestamps integer[]
 
+---@alias FrecencyDatabaseVersion "v1"
+
 ---@class FrecencyDatabase
----@field tx FrecencyPlenaryAsyncControlChannelTx
----@field private file_lock FrecencyFileLock
----@field private filename string
----@field private fs FrecencyFS
+---@field private _file_lock FrecencyFileLock
+---@field private file_lock_rx async fun(): ...
+---@field private file_lock_tx fun(...): nil
 ---@field private tbl FrecencyDatabaseTable
----@field private version "v1"
+---@field private version FrecencyDatabaseVersion
+---@field private watcher_rx FrecencyPlenaryAsyncControlChannelRx
+---@field private watcher_tx FrecencyPlenaryAsyncControlChannelTx
 local Database = {}
 
----@param fs FrecencyFS
 ---@return FrecencyDatabase
-Database.new = function(fs)
+Database.new = function()
   local version = "v1"
-  local self = setmetatable({
-    fs = fs,
+  local file_lock_tx, file_lock_rx = async.control.channel.oneshot()
+  local watcher_tx, watcher_rx = async.control.channel.mpsc()
+  return setmetatable({
+    file_lock_rx = file_lock_rx,
+    file_lock_tx = file_lock_tx,
     tbl = Table.new(version),
     version = version,
+    watcher_rx = watcher_rx,
+    watcher_tx = watcher_tx,
   }, { __index = Database })
-  self.filename = (function()
+end
+
+---@async
+---@return string
+function Database:filename()
+  local file_v1 = "file_frecency.bin"
+
+  ---@async
+  ---@return string
+  local function filename_v1()
     -- NOTE: for backward compatibility
     -- If the user does not set db_root specifically, search DB in
     -- $XDG_DATA_HOME/nvim in addition to $XDG_STATE_HOME/nvim (default value).
-    local file = "file_frecency.bin"
-    local db = Path.new(config.db_root, file)
-    if not config.ext_config.db_root and not db:exists() then
-      local old_location = Path.new(vim.fn.stdpath "data", file)
-      if old_location:exists() then
-        return old_location.filename
+    local db = Path.new(config.db_root, file_v1).filename
+    if not config.ext_config.db_root and not fs.exists(db) then
+      local old_location = Path.new(vim.fn.stdpath "data", file_v1).filename
+      if fs.exists(old_location) then
+        return old_location
       end
     end
-    return db.filename
-  end)()
-  self.file_lock = FileLock.new(self.filename)
-  local rx
-  self.tx, rx = async.control.channel.mpsc()
-  self.tx.send "load"
-  watcher.watch(self.filename, function()
-    self.tx.send "load"
+    return db
+  end
+
+  if self.version == "v1" then
+    return filename_v1()
+  else
+    error(("unknown version: %s"):format(self.version))
+  end
+end
+
+---@async
+---@return nil
+function Database:start()
+  local target = self:filename()
+  self.file_lock_tx(FileLock.new(target))
+  self.watcher_tx.send "load"
+  watcher.watch(target, function()
+    self.watcher_tx.send "load"
   end)
   async.void(function()
     while true do
-      local mode = rx.recv()
+      local mode = self.watcher_rx.recv()
       log.debug("DB coroutine start:", mode)
       if mode == "load" then
         self:load()
@@ -66,14 +92,15 @@ Database.new = function(fs)
       log.debug("DB coroutine end:", mode)
     end
   end)()
-  return self
 end
 
+---@async
 ---@return boolean
 function Database:has_entry()
   return not vim.tbl_isempty(self.tbl.records)
 end
 
+---@async
 ---@param paths string[]
 ---@return nil
 function Database:insert_files(paths)
@@ -83,28 +110,32 @@ function Database:insert_files(paths)
   for _, path in ipairs(paths) do
     self.tbl.records[path] = { count = 1, timestamps = { 0 } }
   end
-  self.tx.send "save"
+  self.watcher_tx.send "save"
 end
 
+---@async
 ---@return string[]
 function Database:unlinked_entries()
-  local paths = {}
-  for file in pairs(self.tbl.records) do
-    if not self.fs:is_valid_path(file) then
-      table.insert(paths, file)
+  return vim.tbl_flatten(async.util.join(vim.tbl_map(function(path)
+    return function()
+      local err, realpath = async.uv.fs_realpath(path)
+      if err or not realpath or realpath ~= path or fs.is_ignored(realpath) then
+        return path
+      end
     end
-  end
-  return paths
+  end, vim.tbl_keys(self.tbl.records))))
 end
 
+---@async
 ---@param paths string[]
 function Database:remove_files(paths)
   for _, file in ipairs(paths) do
     self.tbl.records[file] = nil
   end
-  self.tx.send "save"
+  self.watcher_tx.send "save"
 end
 
+---@async
 ---@param path string
 ---@param epoch? integer
 function Database:update(path, epoch)
@@ -120,9 +151,10 @@ function Database:update(path, epoch)
     record.timestamps = new_table
   end
   self.tbl.records[path] = record
-  self.tx.send "save"
+  self.watcher_tx.send "save"
 end
 
+---@async
 ---@param workspace? string
 ---@param epoch? integer
 ---@return FrecencyDatabaseEntry[]
@@ -130,7 +162,7 @@ function Database:get_entries(workspace, epoch)
   local now = epoch or os.time()
   local items = {}
   for path, record in pairs(self.tbl.records) do
-    if self.fs:starts_with(path, workspace) then
+    if fs.starts_with(path, workspace) then
       table.insert(items, {
         path = path,
         count = record.count,
@@ -148,13 +180,13 @@ end
 ---@return nil
 function Database:load()
   local start = os.clock()
-  local err, data = self.file_lock:with(function()
-    local err, stat = async.uv.fs_stat(self.filename)
+  local err, data = self:file_lock():with(function(target)
+    local err, stat = async.uv.fs_stat(target)
     if err then
       return nil
     end
     local fd
-    err, fd = async.uv.fs_open(self.filename, "r", tonumber("644", 8))
+    err, fd = async.uv.fs_open(target, "r", tonumber("644", 8))
     assert(not err, err)
     local data
     err, data = async.uv.fs_read(fd, stat.size)
@@ -173,9 +205,9 @@ end
 ---@return nil
 function Database:save()
   local start = os.clock()
-  local err = self.file_lock:with(function()
-    self:raw_save(self.tbl:raw())
-    local err, stat = async.uv.fs_stat(self.filename)
+  local err = self:file_lock():with(function(target)
+    self:raw_save(self.tbl:raw(), target)
+    local err, stat = async.uv.fs_stat(target)
     assert(not err, err)
     watcher.update(stat)
     return nil
@@ -185,16 +217,18 @@ function Database:save()
 end
 
 ---@async
+---@param target string
 ---@param tbl FrecencyDatabaseRawTable
-function Database:raw_save(tbl)
+function Database:raw_save(tbl, target)
   local f = assert(load("return " .. vim.inspect(tbl)))
   local data = string.dump(f)
-  local err, fd = async.uv.fs_open(self.filename, "w", tonumber("644", 8))
+  local err, fd = async.uv.fs_open(target, "w", tonumber("644", 8))
   assert(not err, err)
   assert(not async.uv.fs_write(fd, data))
   assert(not async.uv.fs_close(fd))
 end
 
+---@async
 ---@param path string
 ---@return boolean
 function Database:remove_entry(path)
@@ -202,8 +236,18 @@ function Database:remove_entry(path)
     return false
   end
   self.tbl.records[path] = nil
-  self.tx.send "save"
+  self.watcher_tx.send "save"
   return true
 end
 
+---@private
+---@async
+---@return FrecencyFileLock
+function Database:file_lock()
+  if not self._file_lock then
+    self._file_lock = self.file_lock_rx()
+  end
+  return self._file_lock
+end
+
 return Database
diff --git a/lua/frecency/database/table.lua b/lua/frecency/database/table.lua
index f461f66..8618ca2 100644
--- a/lua/frecency/database/table.lua
+++ b/lua/frecency/database/table.lua
@@ -1,4 +1,5 @@
 local log = require "frecency.log"
+local async = require "plenary.async"
 
 ---@class FrecencyDatabaseRecordValue
 ---@field count integer
@@ -18,14 +19,12 @@ Table.new = function(version)
   return setmetatable({ is_ready = false, version = version }, { __index = Table.__index })
 end
 
+---@async
+---@param key string
 function Table:__index(key)
   if key == "records" and not rawget(self, "is_ready") then
-    local start = os.clock()
-    log.debug "waiting start"
     Table.wait_ready(self)
-    log.debug(("waiting until DB become clean takes %f seconds"):format(os.clock() - start))
   end
-  log.debug(("is_ready: %s, key: %s, value: %s"):format(rawget(self, "is_ready"), key, rawget(self, key)))
   return vim.F.if_nil(rawget(self, key), Table[key])
 end
 
@@ -45,11 +44,16 @@ function Table:set(raw_table)
 end
 
 ---This is for internal or testing use only.
+---@async
 ---@return nil
 function Table:wait_ready()
-  vim.wait(2000, function()
-    return rawget(self, "is_ready")
-  end)
+  local start = os.clock()
+  local t = 0.2
+  while not rawget(self, "is_ready") do
+    async.util.sleep(t)
+    t = t * 2
+  end
+  log.debug(("wait_ready() takes %f seconds"):format(os.clock() - start))
 end
 
 return Table
diff --git a/lua/frecency/entry_maker.lua b/lua/frecency/entry_maker.lua
index 3aab98e..2fa4af7 100644
--- a/lua/frecency/entry_maker.lua
+++ b/lua/frecency/entry_maker.lua
@@ -1,19 +1,18 @@
 local WebDevicons = require "frecency.web_devicons"
 local config = require "frecency.config"
+local fs = require "frecency.fs"
 local Path = require "plenary.path" --[[@as FrecencyPlenaryPath]]
 local entry_display = require "telescope.pickers.entry_display" --[[@as FrecencyTelescopeEntryDisplay]]
 local utils = require "telescope.utils" --[[@as FrecencyTelescopeUtils]]
 
 ---@class FrecencyEntryMaker
----@field fs FrecencyFS
 ---@field loaded table<string,boolean>
 ---@field web_devicons WebDevicons
 local EntryMaker = {}
 
----@param fs FrecencyFS
 ---@return FrecencyEntryMaker
-EntryMaker.new = function(fs)
-  return setmetatable({ fs = fs, web_devicons = WebDevicons.new() }, { __index = EntryMaker })
+EntryMaker.new = function()
+  return setmetatable({ web_devicons = WebDevicons.new() }, { __index = EntryMaker })
 end
 
 ---@class FrecencyEntry
@@ -128,7 +127,7 @@ function EntryMaker:items(entry, workspace, workspace_tag, formatter)
   end
   if config.show_filter_column and workspace and workspace_tag then
     local filtered = self:should_show_tail(workspace_tag) and utils.path_tail(workspace) .. Path.path.sep
-      or self.fs:relative_from_home(workspace) .. Path.path.sep
+      or fs.relative_from_home(workspace) .. Path.path.sep
     table.insert(items, { filtered, "Directory" })
   end
   local formatted_name, path_style = formatter(entry.name)
@@ -153,7 +152,7 @@ end
 ---@return integer
 function EntryMaker:calculate_filter_column_width(workspace, workspace_tag)
   return self:should_show_tail(workspace_tag) and #(utils.path_tail(workspace)) + 1
-    or #(self.fs:relative_from_home(workspace)) + 1
+    or #(fs.relative_from_home(workspace)) + 1
 end
 
 ---@private
diff --git a/lua/frecency/file_lock.lua b/lua/frecency/file_lock.lua
index 738cf0d..3c19ac0 100644
--- a/lua/frecency/file_lock.lua
+++ b/lua/frecency/file_lock.lua
@@ -4,23 +4,28 @@ local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
 
 ---@class FrecencyFileLock
 ---@field base string
----@field config FrecencyFileLockConfig
----@field filename string
+---@field config FrecencyFileLockRawConfig
+---@field lock string
+---@field target string
 local FileLock = {}
 
 ---@class FrecencyFileLockConfig
+---@field retry? integer default: 5
+---@field unlink_retry? integer default: 5
+---@field interval? integer default: 500
+
+---@class FrecencyFileLockRawConfig
 ---@field retry integer default: 5
 ---@field unlink_retry integer default: 5
 ---@field interval integer default: 500
 
----@param path string
+---@param target string
 ---@param file_lock_config? FrecencyFileLockConfig
 ---@return FrecencyFileLock
-FileLock.new = function(path, file_lock_config)
+FileLock.new = function(target, file_lock_config)
+  log.debug(("file_lock new(): %s"):format(target))
   local config = vim.tbl_extend("force", { retry = 5, unlink_retry = 5, interval = 500 }, file_lock_config or {})
-  local self = setmetatable({ config = config }, { __index = FileLock })
-  self.filename = path .. ".lock"
-  return self
+  return setmetatable({ config = config, lock = target .. ".lock", target = target }, { __index = FileLock })
 end
 
 ---@async
@@ -31,21 +36,21 @@ function FileLock:get()
   local err, fd
   while true do
     count = count + 1
-    local dir = Path.new(self.filename):parent()
+    local dir = Path.new(self.lock):parent()
     if not dir:exists() then
       -- TODO: make this call be async
       log.debug(("file_lock get(): mkdir parent: %s"):format(dir.filename))
       ---@diagnostic disable-next-line: undefined-field
       dir:mkdir { parents = true }
     end
-    err, fd = async.uv.fs_open(self.filename, "wx", tonumber("600", 8))
+    err, fd = async.uv.fs_open(self.lock, "wx", tonumber("600", 8))
     if not err then
       break
     end
     async.util.sleep(self.config.interval)
     if count >= self.config.retry then
       log.debug(("file_lock get(): retry count reached. try to delete the lock file: %d"):format(count))
-      err = async.uv.fs_unlink(self.filename)
+      err = async.uv.fs_unlink(self.lock)
       if err then
         log.debug("file_lock get() failed: " .. err)
         unlink_count = unlink_count + 1
@@ -67,12 +72,12 @@ end
 ---@async
 ---@return string? err
 function FileLock:release()
-  local err = async.uv.fs_stat(self.filename)
+  local err = async.uv.fs_stat(self.lock)
   if err then
     log.debug("file_lock release() not found: " .. err)
     return "lock not found"
   end
-  err = async.uv.fs_unlink(self.filename)
+  err = async.uv.fs_unlink(self.lock)
   if err then
     log.debug("file_lock release() unlink failed: " .. err)
     return err
@@ -81,7 +86,7 @@ end
 
 ---@async
 ---@generic T
----@param f fun(): T
+---@param f fun(target: string): T
 ---@return string? err
 ---@return T
 function FileLock:with(f)
@@ -89,7 +94,7 @@ function FileLock:with(f)
   if err then
     return err, nil
   end
-  local ok, result_or_err = pcall(f)
+  local ok, result_or_err = pcall(f, self.target)
   err = self:release()
   if err then
     return err, nil
diff --git a/lua/frecency/finder.lua b/lua/frecency/finder.lua
index bafd9af..6bbccc3 100644
--- a/lua/frecency/finder.lua
+++ b/lua/frecency/finder.lua
@@ -1,4 +1,5 @@
 local config = require "frecency.config"
+local fs = require "frecency.fs"
 local os_util = require "frecency.os_util"
 local log = require "frecency.log"
 local Job = require "plenary.job"
@@ -10,7 +11,6 @@ local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
 ---@field entries FrecencyEntry[]
 ---@field scanned_entries FrecencyEntry[]
 ---@field entry_maker FrecencyEntryMakerInstance
----@field fs FrecencyFS
 ---@field path? string
 ---@field private database FrecencyDatabase
 ---@field private rx FrecencyPlenaryAsyncControlChannelRx
@@ -32,14 +32,13 @@ local Finder = {}
 
 ---@param database FrecencyDatabase
 ---@param entry_maker FrecencyEntryMakerInstance
----@param fs FrecencyFS
 ---@param need_scandir boolean
 ---@param path string?
 ---@param recency FrecencyRecency
 ---@param state FrecencyState
 ---@param finder_config? FrecencyFinderConfig
 ---@return FrecencyFinder
-Finder.new = function(database, entry_maker, fs, need_scandir, path, recency, state, finder_config)
+Finder.new = function(database, entry_maker, need_scandir, path, recency, state, finder_config)
   local tx, rx = async.control.channel.mpsc()
   local scan_tx, scan_rx = async.control.channel.mpsc()
   local self = setmetatable({
@@ -47,7 +46,6 @@ Finder.new = function(database, entry_maker, fs, need_scandir, path, recency, st
     closed = false,
     database = database,
     entry_maker = entry_maker,
-    fs = fs,
     path = path,
     recency = recency,
     state = state,
@@ -168,7 +166,7 @@ end
 ---@return nil
 function Finder:scan_dir_lua()
   local count = 0
-  for name in self.fs:scan_dir(self.path) do
+  for name in fs.scan_dir(self.path) do
     if self.closed then
       break
     end
@@ -296,25 +294,34 @@ function Finder:reflow_results()
     return
   end
   async.util.scheduler()
-  local bufnr = picker.results_bufnr
-  local win = picker.results_win
-  if not bufnr or not win or not vim.api.nvim_buf_is_valid(bufnr) or not vim.api.nvim_win_is_valid(win) then
-    return
-  end
-  picker:clear_extra_rows(bufnr)
-  if picker.sorting_strategy == "descending" then
-    local manager = picker.manager
-    if not manager then
+
+  local function reflow()
+    local bufnr = picker.results_bufnr
+    local win = picker.results_win
+    if not bufnr or not win or not vim.api.nvim_buf_is_valid(bufnr) or not vim.api.nvim_win_is_valid(win) then
       return
     end
-    local worst_line = picker:get_row(manager:num_results())
-    local wininfo = vim.fn.getwininfo(win)[1]
-    local bottom = vim.api.nvim_buf_line_count(bufnr)
-    if not self.reflowed or worst_line > wininfo.botline then
-      self.reflowed = true
-      vim.api.nvim_win_set_cursor(win, { bottom, 0 })
+    picker:clear_extra_rows(bufnr)
+    if picker.sorting_strategy == "descending" then
+      local manager = picker.manager
+      if not manager then
+        return
+      end
+      local worst_line = picker:get_row(manager:num_results())
+      local wininfo = vim.fn.getwininfo(win)[1]
+      local bottom = vim.api.nvim_buf_line_count(bufnr)
+      if not self.reflowed or worst_line > wininfo.botline then
+        self.reflowed = true
+        vim.api.nvim_win_set_cursor(win, { bottom, 0 })
+      end
     end
   end
+
+  if vim.in_fast_event() then
+    reflow()
+  else
+    vim.schedule(reflow)
+  end
 end
 
 return Finder
diff --git a/lua/frecency/fs.lua b/lua/frecency/fs.lua
index 6868839..ef57152 100644
--- a/lua/frecency/fs.lua
+++ b/lua/frecency/fs.lua
@@ -2,57 +2,57 @@ local config = require "frecency.config"
 local os_util = require "frecency.os_util"
 local log = require "frecency.log"
 local Path = require "plenary.path" --[[@as FrecencyPlenaryPath]]
+local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
 local scandir = require "plenary.scandir"
 local uv = vim.uv or vim.loop
 
----@class FrecencyFS
----@field os_homedir string
----@field private config FrecencyFSConfig
----@field private ignore_regexes string[]
-local FS = {}
+local M = {
+  os_homedir = assert(uv.os_homedir()),
+}
 
----@class FrecencyFSConfig
----@field scan_depth integer?
+-- TODO: make this configurable
+local SCAN_DEPTH = 100
 
----@param fs_config? FrecencyFSConfig
----@return FrecencyFS
-FS.new = function(fs_config)
-  local self= setmetatable(
-    { config = vim.tbl_extend("force", { scan_depth = 100 }, fs_config or {}), os_homedir = assert(uv.os_homedir()) },
-    { __index = FS }
-  )
-  ---@param pattern string
-  self.ignore_regexes = vim.tbl_map(function(pattern)
-    local regex = vim.pesc(pattern):gsub("%%%*", ".*"):gsub("%%%?", ".")
-    return "^" .. regex .. "$"
-  end, config.ignore_patterns)
-  return self
+---@param path string
+---@return boolean
+function M.is_ignored(path)
+  for _, regex in ipairs(config.ignore_regexes()) do
+    if path:find(regex) then
+      return true
+    end
+  end
+  return false
 end
 
+---@async
 ---@param path? string
 ---@return boolean
-function FS:is_valid_path(path)
-  return not not path and Path:new(path):is_file() and not self:is_ignored(path)
+function M.is_valid_path(path)
+  if not path then
+    return false
+  end
+  local err, st = async.uv.fs_stat(path)
+  return not err and st.type == "file" and not M.is_ignored(path)
 end
 
 ---@param path string
 ---@return function
-function FS:scan_dir(path)
+function M.scan_dir(path)
   log.debug { path = path }
-  local gitignore = self:make_gitignore(path)
+  local gitignore = M.make_gitignore(path)
   return coroutine.wrap(function()
     for name, type in
       vim.fs.dir(path, {
-        depth = self.config.scan_depth,
+        depth = SCAN_DEPTH,
         skip = function(dirname)
-          if self:is_ignored(os_util.join_path(path, dirname)) then
+          if M.is_ignored(os_util.join_path(path, dirname)) then
             return false
           end
         end,
       })
     do
       local fullpath = os_util.join_path(path, name)
-      if type == "file" and not self:is_ignored(fullpath) and gitignore({ path }, fullpath) then
+      if type == "file" and not M.is_ignored(fullpath) and gitignore({ path }, fullpath) then
         coroutine.yield(name)
       end
     end
@@ -61,8 +61,8 @@ end
 
 ---@param path string
 ---@return string
-function FS:relative_from_home(path)
-  return Path:new(path):make_relative(self.os_homedir)
+function M.relative_from_home(path)
+  return Path:new(path):make_relative(M.os_homedir)
 end
 
 ---@type table<string,string>
@@ -71,7 +71,7 @@ local with_sep = {}
 ---@param path string
 ---@param base? string
 ---@return boolean
-function FS:starts_with(path, base)
+function M.starts_with(path, base)
   if not base then
     return true
   end
@@ -81,25 +81,20 @@ function FS:starts_with(path, base)
   return path:find(with_sep[base], 1, true) == 1
 end
 
----@private
+---@async
 ---@param path string
 ---@return boolean
-function FS:is_ignored(path)
-  for _, regex in ipairs(self.ignore_regexes) do
-    if path:find(regex) then
-      return true
-    end
-  end
-  return false
+function M.exists(path)
+  return not (async.uv.fs_stat(path))
 end
 
 ---@private
 ---@param basepath string
 ---@return fun(base_paths: string[], entry: string): boolean
-function FS:make_gitignore(basepath)
+function M.make_gitignore(basepath)
   return scandir.__make_gitignore { basepath } or function(_, _)
     return true
   end
 end
 
-return FS
+return M
diff --git a/lua/frecency/init.lua b/lua/frecency/init.lua
index bdc0eb3..b83d860 100644
--- a/lua/frecency/init.lua
+++ b/lua/frecency/init.lua
@@ -3,11 +3,11 @@
 ---setup() to be initialized.
 ---@class FrecencyInstance
 ---@field complete fun(findstart: 1|0, base: string): integer|''|string[]
----@field delete fun(path: string): nil
+---@field delete async fun(path: string): nil
 ---@field query fun(opts?: FrecencyQueryOpts): FrecencyQueryEntry[]|string[]
 ---@field register fun(bufnr: integer, datetime: string?): nil
 ---@field start fun(opts: FrecencyPickerOptions?): nil
----@field validate_database fun(force: boolean?): nil
+---@field validate_database async fun(force: boolean?): nil
 local frecency = setmetatable({}, {
   ---@param self FrecencyInstance
   ---@param key "complete"|"delete"|"register"|"start"|"validate_database"
@@ -28,6 +28,10 @@ local frecency = setmetatable({}, {
   end,
 })
 
+local function async_call(f, ...)
+  require("plenary.async").void(f)(...)
+end
+
 local setup_done = false
 
 ---When this func is called, Frecency instance is NOT created but only
@@ -46,15 +50,20 @@ local function setup(ext_config)
   vim.api.nvim_set_hl(0, "TelescopeFrecencyScores", { link = "Number", default = true })
   vim.api.nvim_set_hl(0, "TelescopeQueryFilter", { link = "WildMenu", default = true })
 
-  ---@param cmd_info { bang: boolean }
+  ---@class FrecencyCommandInfo
+  ---@field args string
+  ---@field bang boolean
+
+  ---@param cmd_info FrecencyCommandInfo
   vim.api.nvim_create_user_command("FrecencyValidate", function(cmd_info)
-    frecency.validate_database(cmd_info.bang)
+    async_call(frecency.validate_database, cmd_info.bang)
   end, { bang = true, desc = "Clean up DB for telescope-frecency" })
 
-  vim.api.nvim_create_user_command("FrecencyDelete", function(info)
-    local path_string = info.args == "" and "%:p" or info.args
+  ---@param cmd_info FrecencyCommandInfo
+  vim.api.nvim_create_user_command("FrecencyDelete", function(cmd_info)
+    local path_string = cmd_info.args == "" and "%:p" or cmd_info.args
     local path = vim.fn.expand(path_string) --[[@as string]]
-    frecency.delete(path)
+    async_call(frecency.delete, path)
   end, { nargs = "?", complete = "file", desc = "Delete entry from telescope-frecency" })
 
   local group = vim.api.nvim_create_augroup("TelescopeFrecency", {})
diff --git a/lua/frecency/klass.lua b/lua/frecency/klass.lua
index 795d6cf..a192099 100644
--- a/lua/frecency/klass.lua
+++ b/lua/frecency/klass.lua
@@ -1,16 +1,16 @@
 local Database = require "frecency.database"
 local EntryMaker = require "frecency.entry_maker"
-local FS = require "frecency.fs"
 local Picker = require "frecency.picker"
 local Recency = require "frecency.recency"
 local config = require "frecency.config"
+local fs = require "frecency.fs"
 local log = require "frecency.log"
+local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
 
 ---@class Frecency
 ---@field private buf_registered table<integer, boolean> flag to indicate the buffer is registered to the database.
 ---@field private database FrecencyDatabase
 ---@field private entry_maker FrecencyEntryMaker
----@field private fs FrecencyFS
 ---@field private picker FrecencyPicker
 ---@field private recency FrecencyRecency
 local Frecency = {}
@@ -18,9 +18,8 @@ local Frecency = {}
 ---@return Frecency
 Frecency.new = function()
   local self = setmetatable({ buf_registered = {} }, { __index = Frecency }) --[[@as Frecency]]
-  self.fs = FS.new()
-  self.database = Database.new(self.fs)
-  self.entry_maker = EntryMaker.new(self.fs)
+  self.database = Database.new()
+  self.entry_maker = EntryMaker.new()
   self.recency = Recency.new()
   return self
 end
@@ -28,14 +27,29 @@ end
 ---This is called when `:Telescope frecency` is called at the first time.
 ---@return nil
 function Frecency:setup()
-  -- HACK: Wihout this wrapping, it spoils background color detection.
-  -- See https://github.com/nvim-telescope/telescope-frecency.nvim/issues/210
-  vim.defer_fn(function()
+  local done = false
+  ---@async
+  local function init()
+    self.database:start()
     self:assert_db_entries()
     if config.auto_validate then
       self:validate_database()
     end
-  end, 0)
+    done = true
+  end
+
+  local is_async = not not coroutine.running()
+  if is_async then
+    init()
+  else
+    async.void(init)()
+    local ok, status = vim.wait(1000, function()
+      return done
+    end)
+    if not ok then
+      log.error("failed to setup:", status == -1 and "timed out" or "interrupted")
+    end
+  end
 end
 
 ---This can be calledBy `require("telescope").extensions.frecency.frecency`.
@@ -52,7 +66,7 @@ function Frecency:start(opts)
   if opts.hide_current_buffer or config.hide_current_buffer then
     ignore_filenames = { vim.api.nvim_buf_get_name(0) }
   end
-  self.picker = Picker.new(self.database, self.entry_maker, self.fs, self.recency, {
+  self.picker = Picker.new(self.database, self.entry_maker, self.recency, {
     editing_bufnr = vim.api.nvim_get_current_buf(),
     ignore_filenames = ignore_filenames,
     initial_workspace_tag = opts.workspace,
@@ -69,16 +83,7 @@ function Frecency:complete(findstart, base)
   return self.picker:complete(findstart, base)
 end
 
----@private
----@return nil
-function Frecency:assert_db_entries()
-  if not self.database:has_entry() then
-    self.database:insert_files(vim.v.oldfiles)
-    self:notify("Imported %d entries from oldfiles.", #vim.v.oldfiles)
-  end
-end
-
----@private
+---@async
 ---@param force? boolean
 ---@return nil
 function Frecency:validate_database(force)
@@ -110,20 +115,38 @@ function Frecency:validate_database(force)
   end)
 end
 
+---@private
+---@async
+---@return nil
+function Frecency:assert_db_entries()
+  if not self.database:has_entry() then
+    self.database:insert_files(vim.v.oldfiles)
+    self:notify("Imported %d entries from oldfiles.", #vim.v.oldfiles)
+  end
+end
+
 ---@param bufnr integer
 ---@param epoch? integer
 function Frecency:register(bufnr, epoch)
-  if config.ignore_register and config.ignore_register(bufnr) then
+  if (config.ignore_register and config.ignore_register(bufnr)) or self.buf_registered[bufnr] then
     return
   end
   local path = vim.api.nvim_buf_get_name(bufnr)
-  if self.buf_registered[bufnr] or not self.fs:is_valid_path(path) then
-    return
-  end
-  self.database:update(path, epoch)
-  self.buf_registered[bufnr] = true
+  async.void(function()
+    if not fs.is_valid_path(path) then
+      return
+    end
+    local err, realpath = async.uv.fs_realpath(path)
+    if err or not realpath then
+      return
+    end
+    self.database:update(realpath, epoch)
+    self.buf_registered[bufnr] = true
+    log.debug("registered:", bufnr, path)
+  end)()
 end
 
+---@async
 ---@param path string
 ---@return nil
 function Frecency:delete(path)
diff --git a/lua/frecency/picker.lua b/lua/frecency/picker.lua
index f28425d..297b4a5 100644
--- a/lua/frecency/picker.lua
+++ b/lua/frecency/picker.lua
@@ -1,6 +1,7 @@
 local State = require "frecency.state"
 local Finder = require "frecency.finder"
 local config = require "frecency.config"
+local fs = require "frecency.fs"
 local fuzzy_sorter = require "frecency.fuzzy_sorter"
 local substr_sorter = require "frecency.substr_sorter"
 local log = require "frecency.log"
@@ -15,7 +16,6 @@ local uv = vim.loop or vim.uv
 ---@field private config FrecencyPickerConfig
 ---@field private database FrecencyDatabase
 ---@field private entry_maker FrecencyEntryMaker
----@field private fs FrecencyFS
 ---@field private lsp_workspaces string[]
 ---@field private namespace integer
 ---@field private recency FrecencyRecency
@@ -38,16 +38,14 @@ local Picker = {}
 
 ---@param database FrecencyDatabase
 ---@param entry_maker FrecencyEntryMaker
----@param fs FrecencyFS
 ---@param recency FrecencyRecency
 ---@param picker_config FrecencyPickerConfig
 ---@return FrecencyPicker
-Picker.new = function(database, entry_maker, fs, recency, picker_config)
+Picker.new = function(database, entry_maker, recency, picker_config)
   local self = setmetatable({
     config = picker_config,
     database = database,
     entry_maker = entry_maker,
-    fs = fs,
     lsp_workspaces = {},
     namespace = vim.api.nvim_create_namespace "frecency",
     recency = recency,
@@ -80,7 +78,6 @@ function Picker:finder(opts, workspace, workspace_tag)
   return Finder.new(
     self.database,
     entry_maker,
-    self.fs,
     need_scandir,
     workspace,
     self.recency,
@@ -159,8 +156,8 @@ end
 function Picker:default_path_display(opts, path)
   local filename = Path:new(path):make_relative(opts.cwd)
   if not self.workspace then
-    if vim.startswith(filename, self.fs.os_homedir) then
-      filename = "~" .. Path.path.sep .. self.fs:relative_from_home(filename)
+    if vim.startswith(filename, fs.os_homedir) then
+      filename = "~" .. Path.path.sep .. fs.relative_from_home(filename)
     elseif filename ~= path then
       filename = "." .. Path.path.sep .. filename
     end
@@ -269,7 +266,7 @@ function Picker:filepath_formatter(picker_opts)
     for k, v in pairs(picker_opts) do
       opts[k] = v
     end
-    opts.cwd = workspace or self.fs.os_homedir
+    opts.cwd = workspace or fs.os_homedir
 
     return function(filename)
       return utils.transform_path(opts, filename)
diff --git a/lua/frecency/tests/database_spec.lua b/lua/frecency/tests/database_spec.lua
index 1e8e05e..18d98cb 100644
--- a/lua/frecency/tests/database_spec.lua
+++ b/lua/frecency/tests/database_spec.lua
@@ -1,27 +1,18 @@
-local FS = require "frecency.fs"
 local Database = require "frecency.database"
 local config = require "frecency.config"
 local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
 local util = require "frecency.tests.util"
 async.tests.add_to_env()
 
----@param datetime string?
----@return integer
-local function make_epoch(datetime)
-  if not datetime then
-    return os.time()
-  end
-  local tz_fix = datetime:gsub("+(%d%d):(%d%d)$", "+%1%2")
-  return util.time_piece(tz_fix)
-end
+local make_epoch = util.make_epoch
 
 local function with_database(f)
-  local fs = FS.new {}
   local dir, close = util.tmpdir()
   dir:joinpath("file_frecency.bin"):touch()
   return function()
     config.setup { debug = true, db_root = dir.filename }
-    local database = Database.new(fs)
+    local database = Database.new()
+    database:start()
     f(database)
     close()
   end
@@ -33,7 +24,8 @@ end
 ---@param epoch integer
 ---@return FrecencyEntry[]
 local function save_and_load(database, tbl, epoch)
-  database:raw_save(util.v1_table(tbl))
+  ---@diagnostic disable-next-line: invisible
+  database:raw_save(util.v1_table(tbl), database:file_lock().target)
   async.util.sleep(100)
   local entries = database:get_entries(nil, epoch)
   table.sort(entries, function(a, b)
diff --git a/lua/frecency/tests/file_lock_spec.lua b/lua/frecency/tests/file_lock_spec.lua
index a98bb34..e139372 100644
--- a/lua/frecency/tests/file_lock_spec.lua
+++ b/lua/frecency/tests/file_lock_spec.lua
@@ -41,7 +41,7 @@ a.describe("file_lock", function()
       with_dir(function(filename)
         local fl = FileLock.new(filename, { retry = 1, interval = 10 })
         a.it("gets successfully", function()
-          local err, fd = async.uv.fs_open(fl.filename, "wx", tonumber("600", 8))
+          local err, fd = async.uv.fs_open(fl.lock, "wx", tonumber("600", 8))
           assert.is.Nil(err)
           assert.is.Nil(async.uv.fs_close(fd))
           assert.is.Nil(fl:get())
@@ -131,7 +131,7 @@ a.describe("file_lock", function()
           assert.are.same(
             "lock not found",
             fl:with(function()
-              assert.is.Nil(async.uv.fs_unlink(fl.filename))
+              assert.is.Nil(async.uv.fs_unlink(fl.lock))
               return nil
             end)
           )
diff --git a/lua/frecency/tests/frecency_spec.lua b/lua/frecency/tests/frecency_spec.lua
index bd4f1a3..41a807c 100644
--- a/lua/frecency/tests/frecency_spec.lua
+++ b/lua/frecency/tests/frecency_spec.lua
@@ -2,112 +2,14 @@
 -- https://github.com/nvim-lua/plenary.nvim/blob/663246936325062427597964d81d30eaa42ab1e4/lua/plenary/test_harness.lua#L86-L86
 vim.opt.runtimepath:append(vim.env.TELESCOPE_PATH)
 
----@diagnostic disable: invisible, undefined-field
-local Frecency = require "frecency.klass"
-local Picker = require "frecency.picker"
 local util = require "frecency.tests.util"
 local log = require "plenary.log"
-local Path = require "plenary.path"
-local config = require "frecency.config"
 
----@param datetime string?
----@return integer
-local function make_epoch(datetime)
-  if not datetime then
-    return os.time()
-  end
-  local tz_fix = datetime:gsub("+(%d%d):(%d%d)$", "+%1%2")
-  return util.time_piece(tz_fix)
-end
-
----@param files string[]
----@param cb_or_config table|fun(frecency: Frecency, finder: FrecencyFinder, dir: FrecencyPlenaryPath): nil
----@param callback? fun(frecency: Frecency, finder: FrecencyFinder, dir: FrecencyPlenaryPath): nil
----@return nil
-local function with_files(files, cb_or_config, callback)
-  local dir, close = util.make_tree(files)
-  local cfg
-  if type(cb_or_config) == "table" then
-    cfg = vim.tbl_extend("force", { debug = true, db_root = dir.filename }, cb_or_config)
-  else
-    cfg = { debug = true, db_root = dir.filename }
-    callback = cb_or_config
-  end
-  assert(callback)
-  log.debug(cfg)
-  config.setup(cfg)
-  local frecency = Frecency.new()
-  frecency.database.tbl:wait_ready()
-  frecency.picker =
-    Picker.new(frecency.database, frecency.entry_maker, frecency.fs, frecency.recency, { editing_bufnr = 0 })
-  local finder = frecency.picker:finder {}
-  callback(frecency, finder, dir)
-  close()
-end
-
-local function filepath(dir, file)
-  return dir:joinpath(file):absolute()
-end
-
----@param frecency Frecency
----@param dir FrecencyPlenaryPath
----@return fun(file: string, epoch: integer, reset: boolean?): nil
-local function make_register(frecency, dir)
-  return function(file, epoch, reset)
-    local path = filepath(dir, file)
-    vim.cmd.edit(path)
-    local bufnr = assert(vim.fn.bufnr(path))
-    if reset then
-      frecency.buf_registered[bufnr] = nil
-    end
-    frecency:register(bufnr, epoch)
-  end
-end
-
----@param frecency Frecency
----@param dir FrecencyPlenaryPath
----@param callback fun(register: fun(file: string, epoch?: integer): nil): nil
----@return nil
-local function with_fake_register(frecency, dir, callback)
-  local bufnr = 0
-  local buffers = {}
-  local original_nvim_buf_get_name = vim.api.nvim_buf_get_name
-  ---@diagnostic disable-next-line: redefined-local, duplicate-set-field
-  vim.api.nvim_buf_get_name = function(bufnr)
-    return buffers[bufnr]
-  end
-  ---@param file string
-  ---@param epoch integer
-  local function register(file, epoch)
-    local path = filepath(dir, file)
-    Path.new(path):touch()
-    bufnr = bufnr + 1
-    buffers[bufnr] = path
-    frecency:register(bufnr, epoch)
-  end
-  callback(register)
-  vim.api.nvim_buf_get_name = original_nvim_buf_get_name
-end
-
----@param choice "y"|"n"
----@param callback fun(called: fun(): integer): nil
----@return nil
-local function with_fake_vim_ui_select(choice, callback)
-  local original_vim_ui_select = vim.ui.select
-  local count = 0
-  local function called()
-    return count
-  end
-  ---@diagnostic disable-next-line: duplicate-set-field
-  vim.ui.select = function(_, opts, on_choice)
-    count = count + 1
-    log.info(opts.prompt)
-    log.info(opts.format_item(choice))
-    on_choice(choice)
-  end
-  callback(called)
-  vim.ui.select = original_vim_ui_select
-end
+local filepath = util.filepath
+local make_epoch = util.make_epoch
+local make_register = util.make_register
+local with_fake_register = util.with_fake_register
+local with_files = util.with_files
 
 describe("frecency", function()
   describe("register", function()
@@ -116,7 +18,10 @@ describe("frecency", function()
         local register = make_register(frecency, dir)
         local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
         local epoch2 = make_epoch "2023-07-29T01:00:00+09:00"
+        -- HACK: This suspicious 'swapfile' setting is for avoiding E303.
+        vim.o.swapfile = false
         register("hoge1.txt", epoch1)
+        vim.o.swapfile = true
         register("hoge2.txt", epoch2)
 
         it("has valid records in DB", function()
@@ -301,208 +206,6 @@ describe("frecency", function()
     end)
   end)
 
-  describe("validate_database", function()
-    describe("when no files are unlinked", function()
-      with_files({ "hoge1.txt", "hoge2.txt" }, function(frecency, finder, dir)
-        local register = make_register(frecency, dir)
-        local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
-        local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
-        register("hoge1.txt", epoch1)
-        register("hoge2.txt", epoch2)
-
-        it("removes no entries", function()
-          local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
-          assert.are.same({
-            { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
-            { count = 1, path = filepath(dir, "hoge1.txt"), score = 10, timestamps = { epoch1 } },
-          }, results)
-        end)
-      end)
-    end)
-
-    describe("when with not force", function()
-      describe("when files are unlinked but it is less than threshold", function()
-        with_files(
-          { "hoge1.txt", "hoge2.txt", "hoge3.txt", "hoge4.txt", "hoge5.txt" },
-          { db_validate_threshold = 3 },
-          function(frecency, finder, dir)
-            local register = make_register(frecency, dir)
-            local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
-            local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
-            local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
-            local epoch4 = make_epoch "2023-07-29T00:03:00+09:00"
-            local epoch5 = make_epoch "2023-07-29T00:04:00+09:00"
-            register("hoge1.txt", epoch1)
-            register("hoge2.txt", epoch2)
-            register("hoge3.txt", epoch3)
-            register("hoge4.txt", epoch4)
-            register("hoge5.txt", epoch5)
-            dir:joinpath("hoge1.txt"):rm()
-            dir:joinpath("hoge2.txt"):rm()
-            frecency:validate_database()
-
-            it("removes no entries", function()
-              local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
-              table.sort(results, function(a, b)
-                return a.path < b.path
-              end)
-              assert.are.same({
-                { count = 1, path = filepath(dir, "hoge1.txt"), score = 10, timestamps = { epoch1 } },
-                { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
-                { count = 1, path = filepath(dir, "hoge3.txt"), score = 10, timestamps = { epoch3 } },
-                { count = 1, path = filepath(dir, "hoge4.txt"), score = 10, timestamps = { epoch4 } },
-                { count = 1, path = filepath(dir, "hoge5.txt"), score = 10, timestamps = { epoch5 } },
-              }, results)
-            end)
-          end
-        )
-      end)
-
-      describe("when files are unlinked and it is more than threshold", function()
-        describe('when the user response "yes"', function()
-          with_files(
-            { "hoge1.txt", "hoge2.txt", "hoge3.txt", "hoge4.txt", "hoge5.txt" },
-            { db_validate_threshold = 3 },
-            function(frecency, finder, dir)
-              local register = make_register(frecency, dir)
-              local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
-              local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
-              local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
-              local epoch4 = make_epoch "2023-07-29T00:03:00+09:00"
-              local epoch5 = make_epoch "2023-07-29T00:04:00+09:00"
-              register("hoge1.txt", epoch1)
-              register("hoge2.txt", epoch2)
-              register("hoge3.txt", epoch3)
-              register("hoge4.txt", epoch4)
-              register("hoge5.txt", epoch5)
-              dir:joinpath("hoge1.txt"):rm()
-              dir:joinpath("hoge2.txt"):rm()
-              dir:joinpath("hoge3.txt"):rm()
-
-              with_fake_vim_ui_select("y", function(called)
-                frecency:validate_database()
-
-                it("called vim.ui.select()", function()
-                  assert.are.same(1, called())
-                end)
-              end)
-
-              it("removes entries", function()
-                local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
-                table.sort(results, function(a, b)
-                  return a.path < b.path
-                end)
-                assert.are.same({
-                  { count = 1, path = filepath(dir, "hoge4.txt"), score = 10, timestamps = { epoch4 } },
-                  { count = 1, path = filepath(dir, "hoge5.txt"), score = 10, timestamps = { epoch5 } },
-                }, results)
-              end)
-            end
-          )
-        end)
-
-        describe('when the user response "no"', function()
-          with_files(
-            { "hoge1.txt", "hoge2.txt", "hoge3.txt", "hoge4.txt", "hoge5.txt" },
-            { db_validate_threshold = 3 },
-            function(frecency, finder, dir)
-              local register = make_register(frecency, dir)
-              local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
-              local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
-              local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
-              local epoch4 = make_epoch "2023-07-29T00:03:00+09:00"
-              local epoch5 = make_epoch "2023-07-29T00:04:00+09:00"
-              register("hoge1.txt", epoch1)
-              register("hoge2.txt", epoch2)
-              register("hoge3.txt", epoch3)
-              register("hoge4.txt", epoch4)
-              register("hoge5.txt", epoch5)
-              dir:joinpath("hoge1.txt"):rm()
-              dir:joinpath("hoge2.txt"):rm()
-              dir:joinpath("hoge3.txt"):rm()
-
-              with_fake_vim_ui_select("n", function(called)
-                frecency:validate_database()
-
-                it("called vim.ui.select()", function()
-                  assert.are.same(1, called())
-                end)
-              end)
-
-              it("removes no entries", function()
-                local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
-                table.sort(results, function(a, b)
-                  return a.path < b.path
-                end)
-                assert.are.same({
-                  { count = 1, path = filepath(dir, "hoge1.txt"), score = 10, timestamps = { epoch1 } },
-                  { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
-                  { count = 1, path = filepath(dir, "hoge3.txt"), score = 10, timestamps = { epoch3 } },
-                  { count = 1, path = filepath(dir, "hoge4.txt"), score = 10, timestamps = { epoch4 } },
-                  { count = 1, path = filepath(dir, "hoge5.txt"), score = 10, timestamps = { epoch5 } },
-                }, results)
-              end)
-            end
-          )
-        end)
-      end)
-    end)
-
-    describe("when with force", function()
-      describe("when db_safe_mode is true", function()
-        with_files({ "hoge1.txt", "hoge2.txt" }, function(frecency, finder, dir)
-          local register = make_register(frecency, dir)
-          local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
-          local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
-          register("hoge1.txt", epoch1)
-          register("hoge2.txt", epoch2)
-          dir:joinpath("hoge1.txt"):rm()
-
-          with_fake_vim_ui_select("y", function(called)
-            frecency:validate_database(true)
-
-            it("called vim.ui.select()", function()
-              assert.are.same(1, called())
-            end)
-          end)
-
-          it("needs confirmation for removing entries", function()
-            local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
-            assert.are.same({
-              { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
-            }, results)
-          end)
-        end)
-      end)
-
-      describe("when db_safe_mode is false", function()
-        with_files({ "hoge1.txt", "hoge2.txt" }, { db_safe_mode = false }, function(frecency, finder, dir)
-          local register = make_register(frecency, dir)
-          local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
-          local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
-          register("hoge1.txt", epoch1)
-          register("hoge2.txt", epoch2)
-          dir:joinpath("hoge1.txt"):rm()
-
-          with_fake_vim_ui_select("y", function(called)
-            frecency:validate_database(true)
-
-            it("did not call vim.ui.select()", function()
-              assert.are.same(0, called())
-            end)
-          end)
-
-          it("needs no confirmation for removing entries", function()
-            local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
-            assert.are.same({
-              { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
-            }, results)
-          end)
-        end)
-      end)
-    end)
-  end)
-
   describe("delete", function()
     describe("when file exists", function()
       with_files({ "hoge1.txt", "hoge2.txt" }, function(frecency, finder, dir)
@@ -515,8 +218,9 @@ describe("frecency", function()
         it("deletes the file successfully", function()
           local path = filepath(dir, "hoge2.txt")
           local result
-          ---@diagnostic disable-next-line: duplicate-set-field
+          ---@diagnostic disable-next-line: duplicate-set-field, invisible
           frecency.notify = function(self, fmt, ...)
+            ---@diagnostic disable-next-line: invisible
             vim.notify(self:message(fmt, ...))
             result = true
           end
diff --git a/lua/frecency/tests/frecency_validate_database_spec.lua b/lua/frecency/tests/frecency_validate_database_spec.lua
new file mode 100644
index 0000000..c1129de
--- /dev/null
+++ b/lua/frecency/tests/frecency_validate_database_spec.lua
@@ -0,0 +1,258 @@
+local util = require "frecency.tests.util"
+local async = require "plenary.async"
+
+local filepath = util.filepath
+local make_epoch = util.make_epoch
+local make_register = util.make_register
+local with_fake_vim_ui_select = util.with_fake_vim_ui_select
+local with_files = util.with_files
+
+-- HACK: avoid error:
+-- E5560: nvim_echo must not be called in a lua loop callback
+vim.notify = function(_, _) end
+
+describe("frecency", function()
+  describe("validate_database", function()
+    describe("when no files are unlinked", function()
+      with_files({ "hoge1.txt", "hoge2.txt" }, function(frecency, finder, dir)
+        local register = make_register(frecency, dir)
+        local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+        local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+        register("hoge1.txt", epoch1)
+        register("hoge2.txt", epoch2)
+
+        it("removes no entries", function()
+          local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
+          assert.are.same({
+            { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
+            { count = 1, path = filepath(dir, "hoge1.txt"), score = 10, timestamps = { epoch1 } },
+          }, results)
+        end)
+      end)
+    end)
+
+    describe("when with not force", function()
+      describe("when files are unlinked but it is less than threshold", function()
+        with_files(
+          { "hoge1.txt", "hoge2.txt", "hoge3.txt", "hoge4.txt", "hoge5.txt" },
+          { db_validate_threshold = 3 },
+          function(frecency, finder, dir)
+            local register = make_register(frecency, dir)
+            local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+            local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+            local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
+            local epoch4 = make_epoch "2023-07-29T00:03:00+09:00"
+            local epoch5 = make_epoch "2023-07-29T00:04:00+09:00"
+            register("hoge1.txt", epoch1)
+            register("hoge2.txt", epoch2)
+            register("hoge3.txt", epoch3)
+            register("hoge4.txt", epoch4)
+            register("hoge5.txt", epoch5)
+            dir:joinpath("hoge1.txt"):rm()
+            dir:joinpath("hoge2.txt"):rm()
+            async.util.block_on(function()
+              frecency:validate_database()
+            end)
+
+            it("removes no entries", function()
+              local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
+              table.sort(results, function(a, b)
+                return a.path < b.path
+              end)
+              assert.are.same({
+                { count = 1, path = filepath(dir, "hoge1.txt"), score = 10, timestamps = { epoch1 } },
+                { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
+                { count = 1, path = filepath(dir, "hoge3.txt"), score = 10, timestamps = { epoch3 } },
+                { count = 1, path = filepath(dir, "hoge4.txt"), score = 10, timestamps = { epoch4 } },
+                { count = 1, path = filepath(dir, "hoge5.txt"), score = 10, timestamps = { epoch5 } },
+              }, results)
+            end)
+          end
+        )
+      end)
+
+      describe("when files are unlinked and it is more than threshold", function()
+        describe('when the user response "yes"', function()
+          with_files(
+            { "hoge1.txt", "hoge2.txt", "hoge3.txt", "hoge4.txt", "hoge5.txt" },
+            { db_validate_threshold = 3 },
+            function(frecency, finder, dir)
+              local register = make_register(frecency, dir)
+              local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+              local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+              local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
+              local epoch4 = make_epoch "2023-07-29T00:03:00+09:00"
+              local epoch5 = make_epoch "2023-07-29T00:04:00+09:00"
+              register("hoge1.txt", epoch1)
+              register("hoge2.txt", epoch2)
+              register("hoge3.txt", epoch3)
+              register("hoge4.txt", epoch4)
+              register("hoge5.txt", epoch5)
+              dir:joinpath("hoge1.txt"):rm()
+              dir:joinpath("hoge2.txt"):rm()
+              dir:joinpath("hoge3.txt"):rm()
+
+              with_fake_vim_ui_select("y", function(called)
+                async.util.block_on(function()
+                  frecency:validate_database()
+                end)
+
+                it("called vim.ui.select()", function()
+                  assert.are.same(1, called())
+                end)
+              end)
+
+              it("removes entries", function()
+                local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
+                table.sort(results, function(a, b)
+                  return a.path < b.path
+                end)
+                assert.are.same({
+                  { count = 1, path = filepath(dir, "hoge4.txt"), score = 10, timestamps = { epoch4 } },
+                  { count = 1, path = filepath(dir, "hoge5.txt"), score = 10, timestamps = { epoch5 } },
+                }, results)
+              end)
+            end
+          )
+        end)
+
+        describe('when the user response "no"', function()
+          with_files(
+            { "hoge1.txt", "hoge2.txt", "hoge3.txt", "hoge4.txt", "hoge5.txt" },
+            { db_validate_threshold = 3 },
+            function(frecency, finder, dir)
+              local register = make_register(frecency, dir)
+              local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+              local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+              local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
+              local epoch4 = make_epoch "2023-07-29T00:03:00+09:00"
+              local epoch5 = make_epoch "2023-07-29T00:04:00+09:00"
+              register("hoge1.txt", epoch1)
+              register("hoge2.txt", epoch2)
+              register("hoge3.txt", epoch3)
+              register("hoge4.txt", epoch4)
+              register("hoge5.txt", epoch5)
+              dir:joinpath("hoge1.txt"):rm()
+              dir:joinpath("hoge2.txt"):rm()
+              dir:joinpath("hoge3.txt"):rm()
+
+              with_fake_vim_ui_select("n", function(called)
+                async.util.block_on(function()
+                  frecency:validate_database()
+                end)
+
+                it("called vim.ui.select()", function()
+                  assert.are.same(1, called())
+                end)
+              end)
+
+              it("removes no entries", function()
+                local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
+                table.sort(results, function(a, b)
+                  return a.path < b.path
+                end)
+                assert.are.same({
+                  { count = 1, path = filepath(dir, "hoge1.txt"), score = 10, timestamps = { epoch1 } },
+                  { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
+                  { count = 1, path = filepath(dir, "hoge3.txt"), score = 10, timestamps = { epoch3 } },
+                  { count = 1, path = filepath(dir, "hoge4.txt"), score = 10, timestamps = { epoch4 } },
+                  { count = 1, path = filepath(dir, "hoge5.txt"), score = 10, timestamps = { epoch5 } },
+                }, results)
+              end)
+            end
+          )
+        end)
+      end)
+    end)
+
+    describe("when with force", function()
+      describe("when db_safe_mode is true", function()
+        with_files({ "hoge1.txt", "hoge2.txt" }, function(frecency, finder, dir)
+          local register = make_register(frecency, dir)
+          local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+          local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+          register("hoge1.txt", epoch1)
+          register("hoge2.txt", epoch2)
+          dir:joinpath("hoge1.txt"):rm()
+
+          with_fake_vim_ui_select("y", function(called)
+            async.util.block_on(function()
+              frecency:validate_database(true)
+            end)
+
+            it("called vim.ui.select()", function()
+              assert.are.same(1, called())
+            end)
+          end)
+
+          it("needs confirmation for removing entries", function()
+            local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
+            assert.are.same({
+              { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
+            }, results)
+          end)
+        end)
+      end)
+
+      describe("when db_safe_mode is false", function()
+        with_files({ "hoge1.txt", "hoge2.txt" }, { db_safe_mode = false }, function(frecency, finder, dir)
+          local register = make_register(frecency, dir)
+          local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+          local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+          register("hoge1.txt", epoch1)
+          register("hoge2.txt", epoch2)
+          dir:joinpath("hoge1.txt"):rm()
+
+          with_fake_vim_ui_select("y", function(called)
+            async.util.block_on(function()
+              frecency:validate_database(true)
+            end)
+
+            it("did not call vim.ui.select()", function()
+              assert.are.same(0, called())
+            end)
+          end)
+
+          it("needs no confirmation for removing entries", function()
+            local results = finder:get_results(nil, make_epoch "2023-07-29T02:00:00+09:00")
+            assert.are.same({
+              { count = 1, path = filepath(dir, "hoge2.txt"), score = 10, timestamps = { epoch2 } },
+            }, results)
+          end)
+        end)
+      end)
+    end)
+
+    describe("when case sensive filename", function()
+      with_files({ "hoge1.txt", "hoge2.txt" }, function(frecency, finder, dir)
+        local register = make_register(frecency, dir)
+        local epoch1 = make_epoch "2023-07-29T00:00:00+09:00"
+        local epoch2 = make_epoch "2023-07-29T00:01:00+09:00"
+        local epoch3 = make_epoch "2023-07-29T00:02:00+09:00"
+        register("hoge1.txt", epoch1)
+        register("hoge2.txt", epoch2, nil, true)
+        dir:joinpath("hoge1.txt"):rm()
+        dir:joinpath("hoge2.txt"):rename { new_name = dir:joinpath("_hoge2.txt").filename }
+        dir:joinpath("_hoge2.txt"):rename { new_name = dir:joinpath("Hoge2.txt").filename }
+        register("Hoge2.txt", epoch3)
+
+        with_fake_vim_ui_select("y", function(called)
+          async.util.block_on(function()
+            frecency:validate_database(true)
+          end)
+
+          it("calls vim.ui.select()", function()
+            assert.are.same(1, called())
+          end)
+        end)
+
+        it("removes duplicated case sensitive filenames", function()
+          local results = finder:get_results(nil, make_epoch "2023-07-29T03:00:00+09:00")
+          assert.are.same({
+            { count = 1, path = filepath(dir, "Hoge2.txt"), score = 10, timestamps = { epoch3 } },
+          }, results)
+        end)
+      end)
+    end)
+  end)
+end)
diff --git a/lua/frecency/tests/util.lua b/lua/frecency/tests/util.lua
index b7c230e..933b396 100644
--- a/lua/frecency/tests/util.lua
+++ b/lua/frecency/tests/util.lua
@@ -1,4 +1,9 @@
+---@diagnostic disable: invisible, undefined-field
+local Frecency = require "frecency.klass"
+local Picker = require "frecency.picker"
+local config = require "frecency.config"
 local uv = vim.uv or vim.loop
+local log = require "plenary.log"
 local async = require "plenary.async" --[[@as FrecencyPlenaryAsync]]
 local Path = require "plenary.path"
 local Job = require "plenary.job"
@@ -7,7 +12,18 @@ local wait = require "frecency.tests.wait"
 ---@return FrecencyPlenaryPath
 ---@return fun(): nil close swwp all entries
 local function tmpdir()
-  local dir = Path:new(Path:new(assert(uv.fs_mkdtemp "tests_XXXXXX")):absolute())
+  local ci = uv.os_getenv "CI"
+  local dir
+  if ci then
+    dir = Path:new(assert(uv.fs_mkdtemp "tests_XXXXXX"))
+  else
+    local tmp = assert(uv.os_tmpdir())
+    -- HACK: plenary.path resolves paths later, so here it resolves in advance.
+    if uv.os_uname().sysname == "Darwin" then
+      tmp = tmp:gsub("^/var", "/private/var")
+    end
+    dir = Path:new(assert(uv.fs_mkdtemp(Path:new(tmp, "tests_XXXXXX").filename)))
+  end
   return dir, function()
     dir:rm { recursive = true }
   end
@@ -37,6 +53,8 @@ local AsyncJob = async.wrap(function(cmd, callback)
 end, 2)
 
 -- NOTE: vim.fn.strptime cannot be used in Lua loop
+---@param iso8601 string
+---@return integer?
 local function time_piece(iso8601)
   local epoch
   wait(function()
@@ -47,9 +65,130 @@ local function time_piece(iso8601)
   return epoch
 end
 
+---@param datetime string?
+---@return integer
+local function make_epoch(datetime)
+  if not datetime then
+    return os.time()
+  end
+  local tz_fix = datetime:gsub("+(%d%d):(%d%d)$", "+%1%2")
+  return time_piece(tz_fix) or 0
+end
+
 ---@param records table<string, FrecencyDatabaseRecordValue>
 local function v1_table(records)
   return { version = "v1", records = records }
 end
 
-return { make_tree = make_tree, tmpdir = tmpdir, v1_table = v1_table, time_piece = time_piece }
+---@param files string[]
+---@param cb_or_config table|fun(frecency: Frecency, finder: FrecencyFinder, dir: FrecencyPlenaryPath): nil
+---@param callback? fun(frecency: Frecency, finder: FrecencyFinder, dir: FrecencyPlenaryPath): nil
+---@return nil
+local function with_files(files, cb_or_config, callback)
+  local dir, close = make_tree(files)
+  local cfg
+  if type(cb_or_config) == "table" then
+    cfg = vim.tbl_extend("force", { debug = true, db_root = dir.filename }, cb_or_config)
+  else
+    cfg = { debug = true, db_root = dir.filename }
+    callback = cb_or_config
+  end
+  assert(callback)
+  log.debug(cfg)
+  config.setup(cfg)
+  local frecency = Frecency.new()
+  async.util.block_on(function()
+    frecency.database:start()
+    frecency.database.tbl:wait_ready()
+  end)
+  frecency.picker = Picker.new(frecency.database, frecency.entry_maker, frecency.recency, { editing_bufnr = 0 })
+  local finder = frecency.picker:finder {}
+  callback(frecency, finder, dir)
+  close()
+end
+
+local function filepath(dir, file)
+  return dir:joinpath(file):absolute()
+end
+
+---@param frecency Frecency
+---@param dir FrecencyPlenaryPath
+---@return fun(file: string, epoch: integer, reset: boolean?, wipeout?: boolean): nil reset: boolean?): nil
+local function make_register(frecency, dir)
+  return function(file, epoch, reset, wipeout)
+    local path = filepath(dir, file)
+    vim.cmd.edit(path)
+    local bufnr = assert(vim.fn.bufnr(path))
+    if reset then
+      frecency.buf_registered[bufnr] = nil
+    end
+    frecency:register(bufnr, epoch)
+    vim.wait(1000, function()
+      return not not frecency.buf_registered[bufnr]
+    end)
+    -- HACK: This is needed because almost the same filenames use the same
+    -- buffer.
+    if wipeout then
+      vim.cmd.bwipeout()
+    end
+  end
+end
+
+---@param frecency Frecency
+---@param dir FrecencyPlenaryPath
+---@param callback fun(register: fun(file: string, epoch?: integer): nil): nil
+---@return nil
+local function with_fake_register(frecency, dir, callback)
+  local bufnr = 0
+  local buffers = {}
+  local original_nvim_buf_get_name = vim.api.nvim_buf_get_name
+  ---@diagnostic disable-next-line: redefined-local, duplicate-set-field
+  vim.api.nvim_buf_get_name = function(bufnr)
+    return buffers[bufnr]
+  end
+  ---@param file string
+  ---@param epoch integer
+  local function register(file, epoch)
+    local path = filepath(dir, file)
+    Path.new(path):touch()
+    bufnr = bufnr + 1
+    buffers[bufnr] = path
+    async.util.block_on(function()
+      frecency:register(bufnr, epoch)
+    end)
+  end
+  callback(register)
+  vim.api.nvim_buf_get_name = original_nvim_buf_get_name
+end
+
+---@param choice "y"|"n"
+---@param callback fun(called: fun(): integer): nil
+---@return nil
+local function with_fake_vim_ui_select(choice, callback)
+  local original_vim_ui_select = vim.ui.select
+  local count = 0
+  local function called()
+    return count
+  end
+  ---@diagnostic disable-next-line: duplicate-set-field
+  vim.ui.select = function(_, opts, on_choice)
+    count = count + 1
+    log.info(opts.prompt)
+    log.info(opts.format_item(choice))
+    on_choice(choice)
+  end
+  callback(called)
+  vim.ui.select = original_vim_ui_select
+end
+
+return {
+  filepath = filepath,
+  make_epoch = make_epoch,
+  make_register = make_register,
+  make_tree = make_tree,
+  tmpdir = tmpdir,
+  v1_table = v1_table,
+  with_fake_register = with_fake_register,
+  with_fake_vim_ui_select = with_fake_vim_ui_select,
+  with_files = with_files,
+}
diff --git a/lua/frecency/types.lua b/lua/frecency/types.lua
index 5154100..88b9cb2 100644
--- a/lua/frecency/types.lua
+++ b/lua/frecency/types.lua
@@ -12,6 +12,7 @@
 ---@field make_relative fun(self: FrecencyPlenaryPath, cwd: string): string
 ---@field parent fun(self: FrecencyPlenaryPath): FrecencyPlenaryPath
 ---@field path { sep: string }
+---@field rename fun(self: FrecencyPlenaryPath, opts: { new_name: string }): nil
 ---@field rm fun(self: FrecencyPlenaryPath, opts?: { recursive: boolean }): nil
 ---@field touch fun(self: FrecencyPlenaryPath, opts?: { parents: boolean }): nil
 
@@ -47,6 +48,11 @@ function FrecencyPlenaryAsync.run(f) end
 ---@class FrecencyPlenaryAsyncControlChannel
 ---@field mpsc fun(): FrecencyPlenaryAsyncControlChannelTx, FrecencyPlenaryAsyncControlChannelRx
 ---@field counter fun(): FrecencyPlenaryAsyncControlChannelTx, FrecencyPlenaryAsyncControlChannelRx
+local FrecencyPlenaryAsyncControlChannel
+
+---@return fun(...): nil tx
+---@return async fun(): ... rx
+function FrecencyPlenaryAsyncControlChannel.oneshot() end
 
 ---@class FrecencyPlenaryAsyncControlChannelTx
 ---@field send fun(entry?: any): nil
@@ -92,6 +98,12 @@ function FrecencyPlenaryAsyncUv.fs_stat(path) end
 ---@return integer fd
 function FrecencyPlenaryAsyncUv.fs_open(path, flags, mode) end
 
+---@async
+---@param path string
+---@return string? err
+---@return string? path
+function FrecencyPlenaryAsyncUv.fs_realpath(path) end
+
 ---@async
 ---@param fd integer
 ---@param size integer
@@ -119,6 +131,11 @@ function FrecencyPlenaryAsyncUv.fs_unlink(path) end
 ---@return string? err
 function FrecencyPlenaryAsyncUv.fs_close(fd) end
 
+---@async
+---@param async_fns (async fun(...): ...)[]
+---@return table
+function FrecencyPlenaryAsyncUtil.join(async_fns) end
+
 ---@async
 ---@param ms integer
 ---@return nil