diff --git a/hbs2-storage-ncq/lib/HBS2/Storage/NCQ2.hs b/hbs2-storage-ncq/lib/HBS2/Storage/NCQ2.hs
index 542ddfbd..1485dffb 100644
--- a/hbs2-storage-ncq/lib/HBS2/Storage/NCQ2.hs
+++ b/hbs2-storage-ncq/lib/HBS2/Storage/NCQ2.hs
@@ -127,6 +127,7 @@ data NCQStorage2 =
   , ncqMemTable       :: Vector  Shard
   , ncqWriteSem       :: TSem
   , ncqWriteQ         :: TVar (Seq HashRef)
+  , ncqStorageTasks   :: TVar Int
   , ncqStorageStopReq :: TVar Bool
   , ncqStorageSyncReq :: TVar Bool
   , ncqSyncNo         :: TVar Int
@@ -156,6 +157,7 @@ ncqStorageOpen2 fp upd = do
   ncqSyncNo         <- newTVarIO 0
   ncqTrackedFiles   <- newTVarIO HPSQ.empty
   ncqCachedEntries  <- newTVarIO 0
+  ncqStorageTasks   <- newTVarIO 0
 
   let ncqSalt       = "EstEFasxrCFqsGDxcY4haFcha9e4ZHRzsPbGUmDfdxLk"
 
@@ -188,6 +190,10 @@ ncqGetWorkDir NCQStorage2{..} = ncqRoot </> show ncqGen
 ncqGetLockFileName :: NCQStorage2 -> FilePath
 ncqGetLockFileName ncq = ncqGetFileName ncq ".lock"
 
+ncqGetNewFossilName :: MonadIO m => NCQStorage2 -> m FilePath
+ncqGetNewFossilName ncq = do
+ liftIO $ emptyTempFile (ncqGetWorkDir ncq) "fossil-.data"
+
 ncqStorageStop2 :: MonadUnliftIO m => NCQStorage2 -> m ()
 ncqStorageStop2 NCQStorage2{..} = do
   atomically $ writeTVar ncqStorageStopReq True
@@ -331,7 +337,7 @@ ncqStorageRun2 ncq@NCQStorage2{..} = flip runContT pure do
     maybe1 what none $ \(fk, fh) ->  do
         closeFd fh
         -- notice $ yellow "indexing" <+> pretty fname
-        idx <- ncqIndexFile ncq (DataFile fk)
+        idx <- ncqRunTaskNoMatterWhat ncq (ncqIndexFile ncq (DataFile fk))
         ncqAddTrackedFile ncq (DataFile fk)
         nwayHashMMapReadOnly idx >>= \case
           Nothing -> err $ "can't open index" <+> pretty idx
@@ -376,9 +382,7 @@ ncqStorageRun2 ncq@NCQStorage2{..} = flip runContT pure do
       rest <- if not (sync || needClose || w > ncqFsync) then
                   pure w
                 else liftIO do
-                  s <- Posix.fileSize <$> Posix.getFdStatus fh
-                  void (appendSection fh (fileTailRecord s))
-                  fileSynchronise fh
+                  appendTailSection fh >> fileSynchronise fh
                   atomically do
                     writeTVar   ncqStorageSyncReq False
                     modifyTVar' ncqSyncNo succ
@@ -420,40 +424,10 @@ ncqStorageRun2 ncq@NCQStorage2{..} = flip runContT pure do
 
     emptyKey = BS.replicate ncqKeyLen 0
 
-    zeroSyncEntry = ncqMakeSectionBS (Just B) zeroHash zeroPayload
-      where zeroPayload = N.bytestring64 0
-            zeroHash    = HashRef (hashObject zeroPayload)
-    {-# INLINE zeroSyncEntry #-}
-
-    zeroSyncEntrySize = fromIntegral (BS.length zeroSyncEntry)
-    {-# INLINE zeroSyncEntrySize #-}
-
-    -- 1. It's B-record
-    -- 2. It's last w64be == fileSize
-    -- 3. It's hash == hash (bytestring64be fileSize)
-    -- 4. recovery-strategy: start-to-end, end-to-start
-    fileTailRecord w = do
-      -- on open: last w64be == fileSize
-      let paylo = N.bytestring64 (fromIntegral w + zeroSyncEntrySize)
-      let h     = hashObject @HbSync paylo & coerce
-      ncqMakeSectionBS (Just M) h paylo
-    {-# INLINE fileTailRecord #-}
-
-    appendSection :: forall m . MonadUnliftIO m
-                => Fd
-                -> ByteString
-                -> m Int -- (FOff, Int)
-
-    appendSection fh section = do
-      -- off <- liftIO $ fdSeek fh SeekFromEnd 0
-      -- pure (fromIntegral off, fromIntegral len)
-      liftIO (Posix.fdWrite fh section) <&> fromIntegral
-
-    {-# INLINE appendSection #-}
 
     openNewDataFile :: forall mx . MonadIO mx => mx (FileKey, Fd)
     openNewDataFile = do
-      fname <-  liftIO $ emptyTempFile (ncqGetWorkDir ncq) "fossil-.data"
+      fname <- ncqGetNewFossilName ncq
       touch fname
       let flags = defaultFileFlags { exclusive = False, creat = Just 0o666 }
       (fromString fname,) <$> liftIO (PosixBase.openFd fname  Posix.ReadWrite flags)
@@ -535,9 +509,14 @@ ncqAddTrackedFile ncq@NCQStorage2{..} fkey = flip runContT pure $ callCC \exit -
   -- FIXME: maybe-creation-time-actually
   let ts = posixToTimeSpec $ PFS.modificationTimeHiRes stat
   let fk = fromString (takeFileName fname)
-  atomically do
+
+  atomically $ ncqAddTrackedFileSTM ncq fk ts
+  pure True
+
+ncqAddTrackedFileSTM :: NCQStorage2 -> FileKey -> TimeSpec -> STM ()
+ncqAddTrackedFileSTM NCQStorage2{..} fk ts = do
     modifyTVar' ncqTrackedFiles (HPSQ.insert fk (FilePrio (Down ts)) Nothing)
-    pure True
+{-# INLINE ncqAddTrackedFileSTM #-}
 
 evictIfNeededSTM :: NCQStorage2 -> Maybe Int -> STM ()
 evictIfNeededSTM NCQStorage2{..} howMany = do
@@ -640,7 +619,202 @@ ncqRepair me@NCQStorage2{} = do
       Right{} -> do
         err $ "skip indexing" <+> pretty dataFile
 
-
 ncqRefHash :: NCQStorage2 -> HashRef -> HashRef
 ncqRefHash NCQStorage2 {..} h = HashRef (hashObject (coerce @_ @ByteString h <> coerce ncqSalt))
 
+ncqRunTaskNoMatterWhat :: MonadUnliftIO m => NCQStorage2 -> m a -> m a
+ncqRunTaskNoMatterWhat NCQStorage2{..} task = do
+  atomically (modifyTVar ncqStorageTasks succ)
+  task `finally` atomically (modifyTVar ncqStorageTasks pred)
+
+ncqRunTask :: MonadUnliftIO m => NCQStorage2 -> a -> m a -> m a
+ncqRunTask ncq@NCQStorage2{..} def task = readTVarIO ncqStorageStopReq >>= \case
+  True -> pure def
+  False -> ncqRunTaskNoMatterWhat ncq task
+
+ncqWaitTasks :: MonadUnliftIO m => NCQStorage2 -> m ()
+ncqWaitTasks NCQStorage2{..} = atomically do
+  tno <- readTVar ncqStorageTasks
+  when (tno > 0) STM.retry
+
+ncqStorageMergeStep :: MonadUnliftIO m => NCQStorage2 -> m Bool
+ncqStorageMergeStep ncq@NCQStorage2{..}  = ncqRunTask ncq False $ flip runContT pure do
+
+  liftIO do
+
+    tracked <- readTVarIO ncqTrackedFiles <&> HPSQ.toList
+    files <- for tracked $ \(f,p,_) -> do
+      let fn = ncqGetFileName ncq (toFileName $ DataFile f)
+      let idx = ncqGetFileName ncq (toFileName $ IndexFile f)
+      sz <- liftIO (fileSize fn)
+      idxHere <- doesFileExist idx
+      pure (f, sz, p, idxHere)
+
+    let bothIdx a b = view _4 a && view _4 b
+
+    let found = flip fix (files, Nothing, Nothing) $ \next -> \case
+          ([], _, r) -> r
+
+          (a:b:rest, Nothing, _) | bothIdx a b -> do
+            next (b:rest, Just (view _2 a + view _2 b), Just (a,b))
+
+          (a:b:rest, Just s, _ ) | bothIdx a b && view _2 a + view _2 b < s -> do
+            next (b:rest, Just (view _2 a + view _2 b), Just (a,b))
+
+          (_:rest, s, r) -> do
+            next (rest, s, r)
+
+    case found of
+      Just (a,b) -> mergeStep a b >> pure True
+      _          -> pure False
+
+  where
+
+    ncqGetNewMergeName :: MonadIO m => NCQStorage2 -> m FilePath
+    ncqGetNewMergeName n@NCQStorage2{} = do
+      let (p,tpl) = splitFileName (ncqGetFileName n "merge-.data")
+      liftIO $ emptyTempFile p tpl
+
+    mergeStep (a,_,p1,_) (b,_,p2,_) = do
+      debug $ "merge" <+> pretty a <+> pretty b
+
+      let fDataNameA  = ncqGetFileName ncq $ toFileName (DataFile a)
+      let fIndexNameA = ncqGetFileName ncq $ toFileName (IndexFile a)
+
+      let fDataNameB  = ncqGetFileName ncq $ toFileName (DataFile b)
+      let fIndexNameB = ncqGetFileName ncq $ toFileName (IndexFile b)
+
+      debug $ "file A" <+> pretty (timeSpecFromFilePrio p1) <+> pretty fDataNameA <+> pretty fIndexNameA
+      debug $ "file B" <+> pretty (timeSpecFromFilePrio p2) <+> pretty fDataNameB <+> pretty fIndexNameB
+
+      doesFileExist fDataNameA   `orFail` ("not exist" <+> pretty fDataNameA)
+      doesFileExist fDataNameB   `orFail` ("not exist" <+> pretty fDataNameB)
+      doesFileExist fIndexNameA  `orFail` ("not exist" <+> pretty fIndexNameA)
+
+      flip runContT pure $ callCC \exit -> do
+
+        mfile <- ncqGetNewMergeName ncq
+
+        ContT $ bracket none $ const do
+          rm mfile
+
+        liftIO $ withBinaryFileAtomic mfile WriteMode $ \fwh -> do
+
+          debug $ "merge: okay, good to go" <+> pretty (takeFileName mfile)
+
+          (mmIdx, nway) <- nwayHashMMapReadOnly fIndexNameA
+                            >>= orThrow (NCQMergeInvariantFailed (show $ "can't mmap" <+> pretty fIndexNameA))
+
+          debug $ "SCAN FILE A" <+> pretty fDataNameA
+
+          writeFiltered ncq fDataNameA fwh $ \_ _ _ _ -> do
+            pure True
+
+          debug $ "SCAN FILE B" <+> pretty fDataNameA
+
+          writeFiltered ncq fDataNameB fwh $ \_ _ k _ -> do
+            foundInA <- liftIO (nwayHashLookup nway mmIdx (coerce k)) <&> isJust
+            let skip = foundInA
+            pure $ not skip
+
+          appendTailSection =<< handleToFd fwh
+
+        liftIO do
+
+          result <- fileSize mfile
+
+          idx <- if result == 0 then
+                    pure Nothing
+                   else do
+                    fossil <- ncqGetNewFossilName ncq
+                    mv mfile fossil
+                    statA <- getFileStatus fDataNameA
+                    let ts = modificationTimeHiRes statA
+                    setFileTimesHiRes fossil ts ts
+                    let fk = DataFile (fromString fossil)
+                    void $ ncqIndexFile ncq fk
+                    pure $ Just (ts,fk)
+
+          atomically do
+            modifyTVar ncqTrackedFiles (HPSQ.delete a)
+            modifyTVar ncqTrackedFiles (HPSQ.delete b)
+            for_ idx $ \(ts,fk) -> do
+              ncqAddTrackedFileSTM ncq (coerce fk) (posixToTimeSpec ts)
+
+          mapM_ rm [fDataNameA, fDataNameB, fIndexNameB, fIndexNameA]
+
+
+    orFail what e = do
+      r <- what
+      unless r (throwIO (NCQMergeInvariantFailed (show e)))
+
+
+writeFiltered :: forall m . MonadIO m
+              => NCQStorage2
+              -> FilePath
+              -> Handle
+              -> ( Integer -> Integer -> HashRef -> ByteString -> m Bool)
+              -> m ()
+
+writeFiltered ncq fn out filt = do
+  ncqStorageScanDataFile ncq fn $ \o s k v -> do
+    skip <- filt o s k v <&> not
+
+    when skip do
+      debug $ pretty k <+> pretty "skipped"
+
+    unless skip $ liftIO do
+      BS.hPut out (LBS.toStrict (makeEntryLBS k v))
+
+  where
+
+    makeEntryLBS h bs = do
+      let b = byteString (coerce @_ @ByteString h)
+               <> byteString bs
+
+      let wbs = toLazyByteString b
+      let len = LBS.length wbs
+      let ws  = byteString (N.bytestring32  (fromIntegral len))
+
+      toLazyByteString (ws <> b)
+
+
+zeroSyncEntry :: ByteString
+zeroSyncEntry = ncqMakeSectionBS (Just B) zeroHash zeroPayload
+  where zeroPayload = N.bytestring64 0
+        zeroHash    = HashRef (hashObject zeroPayload)
+{-# INLINE zeroSyncEntry #-}
+
+zeroSyncEntrySize :: Word64
+zeroSyncEntrySize = fromIntegral (BS.length zeroSyncEntry)
+{-# INLINE zeroSyncEntrySize #-}
+
+-- 1. It's B-record
+-- 2. It's last w64be == fileSize
+-- 3. It's hash == hash (bytestring64be fileSize)
+-- 4. recovery-strategy: start-to-end, end-to-start
+fileTailRecord :: Integral a => a -> ByteString
+fileTailRecord w = do
+  -- on open: last w64be == fileSize
+  let paylo = N.bytestring64 (fromIntegral w + zeroSyncEntrySize)
+  let h     = hashObject @HbSync paylo & coerce
+  ncqMakeSectionBS (Just M) h paylo
+{-# INLINE fileTailRecord #-}
+
+appendSection :: forall m . MonadUnliftIO m
+            => Fd
+            -> ByteString
+            -> m Int -- (FOff, Int)
+
+appendSection fh sect = do
+  -- off <- liftIO $ fdSeek fh SeekFromEnd 0
+  -- pure (fromIntegral off, fromIntegral len)
+  liftIO (Posix.fdWrite fh sect) <&> fromIntegral
+{-# INLINE appendSection #-}
+
+appendTailSection :: MonadIO m => Fd -> m ()
+appendTailSection fh = liftIO do
+  s <- Posix.fileSize <$> Posix.getFdStatus fh
+  void (appendSection fh (fileTailRecord s))
+{-# INLINE appendTailSection #-}
+
diff --git a/hbs2-tests/test/TestNCQ.hs b/hbs2-tests/test/TestNCQ.hs
index 980e8097..4e4c8cc6 100644
--- a/hbs2-tests/test/TestNCQ.hs
+++ b/hbs2-tests/test/TestNCQ.hs
@@ -621,6 +621,99 @@ testNCQ2Simple1 TestEnv{..} = do
       -- debug $ fill 44 (pretty ha) <+> fill 8 (pretty found)
 
 
+genRandomBS :: forall g m . (Monad m, StatefulGen g m) => g -> Int -> m ByteString
+genRandomBS g n = do
+  n <- (`mod` (64*1024)) <$> uniformM @Int g
+  uniformByteStringM n g
+
+sec6 :: RealFrac a => a -> Fixed E6
+sec6 = realToFrac
+
+sec2 :: RealFrac a => a -> Fixed E2
+sec2 = realToFrac
+
+sec3 :: RealFrac a => a -> Fixed E3
+sec3 = realToFrac
+
+testNCQ2Merge1 :: MonadUnliftIO m
+         => Int
+         -> TestEnv
+         -> m ()
+
+testNCQ2Merge1 n TestEnv{..} = do
+  let tmp = testEnvDir
+  let ncqDir   = tmp
+
+  g <- liftIO MWC.createSystemRandom
+
+  let fake = n `div` 3
+
+  ncqWithStorage ncqDir $ \sto -> liftIO do
+
+    notice $ "write" <+> pretty n <+> "random blocks"
+
+    ws <- flip fix (mempty :: HashSet HashRef) $ \loop -> \case
+      hs | HS.size hs >= n -> pure hs
+         | otherwise  -> do
+
+        s <- liftIO $ genRandomBS g (256 * 1024)
+        h <- ncqPutBS sto (Just B) Nothing s
+        loop (HS.insert h hs)
+
+    notice $ "written" <+> pretty (HS.size ws)
+
+    assertBool "all written" (HS.size ws == n)
+
+    nHashes <- HS.fromList . filter (not . flip HS.member ws) <$> replicateM fake do
+                 liftIO (genRandomBS g (64*1024)) <&> HashRef . hashObject
+
+    notice $ "gen" <+> pretty (HS.size nHashes) <+> pretty "missed hashes"
+
+
+    (t1,n1) <- over _2 sum <$> timeItT do
+                  for (HS.toList ws) $ \h -> do
+                    r <- ncqLocate2 sto h
+
+                    unless (isJust r) do
+                      err $ "not found" <+> pretty h
+
+                    pure $ maybe 0 (const 1) r
+
+    notice $ pretty (sec3 t1) <+> pretty n1 <+> pretty (n1 == HS.size ws)
+
+    assertBool "all written" (n1 == HS.size ws)
+
+    ncqWaitTasks sto
+
+    let hashes = HS.toList ws <> HS.toList nHashes
+
+    (t2,_) <- timeItT do
+                for hashes $ \h -> do
+                  r <- ncqLocate2 sto h
+                  pure $ maybe 0 (const 1) r
+
+    notice $ "before-merge" <+> pretty (sec3 t1) <+> pretty (List.length hashes)
+
+    notice $ "merge whatever possible"
+
+    n <- flip fix 0 \next i -> do
+           N2.ncqStorageMergeStep sto >>= \case
+            False -> pure i
+            True  -> next (succ i)
+
+    notice $ "merged" <+> pretty n
+
+    (t3,r) <- timeItT do
+                  for hashes $ \h -> do
+                    ncqLocate2 sto h >>= \case
+                      Nothing -> pure $ Left h
+                      Just{}  -> pure $ Right h
+
+    let w1 = HS.fromList (rights r)
+    let n2 = HS.fromList (lefts r)
+
+    notice $ "after-merge" <+> pretty (sec3 t3) <+> pretty (HS.size w1) <+> pretty (HS.size n2)
+
 testFilterEmulate1 :: MonadUnliftIO m
          => Int
          -> TestEnv
@@ -945,30 +1038,6 @@ main = do
 
           e -> throwIO $ BadFormException @C (mkList e)
 
-        entry $ bindMatch "test:ncq2:concurrent1" $ nil_ $ \case
-          [ LitIntVal tn, LitIntVal n ] -> do
-            debug $ "ncq2:concurrent1" <+> pretty tn <+> pretty n
-            runTest $ testNCQ2Concurrent1 False ( fromIntegral tn) (fromIntegral n)
-
-          e -> throwIO $ BadFormException @C (mkList e)
-
-        entry $ bindMatch "test:ncq2:simple1" $ nil_ $ const $ do
-            runTest testNCQ2Simple1
-
-        entry $ bindMatch "test:ncq2:repair1" $ nil_ $ const $ do
-            runTest testNCQ2Repair1
-
-        entry $ bindMatch "test:ncq2:filefastcheck" $ nil_ $ \case
-          [ StringLike fn ] -> do
-            ncqFileFastCheck fn
-
-          e -> throwIO $ BadFormException @C (mkList e)
-
-        entry $ bindMatch "test:ncq2:concurrent:write:simple1" $ nil_ $ \case
-          [ LitIntVal tn, LitIntVal n ] -> do
-            runTest $ testNCQ2ConcurrentWriteSimple1 ( fromIntegral tn) (fromIntegral n)
-
-          e -> throwIO $ BadFormException @C (mkList e)
 
         entry $ bindMatch "test:ncq:concurrent1:wo" $ nil_ $ \case
           [ LitIntVal tn, LitIntVal n ] -> do
@@ -1005,6 +1074,37 @@ main = do
 
           e -> throwIO $ BadFormException @C (mkList e)
 
+        entry $ bindMatch "test:ncq2:merge1" $ nil_ $ \case
+          [ LitIntVal n ] -> do
+            runTest $ testNCQ2Merge1 (fromIntegral n)
+
+          e -> throwIO $ BadFormException @C (mkList e)
+
+        entry $ bindMatch "test:ncq2:concurrent1" $ nil_ $ \case
+          [ LitIntVal tn, LitIntVal n ] -> do
+            debug $ "ncq2:concurrent1" <+> pretty tn <+> pretty n
+            runTest $ testNCQ2Concurrent1 False ( fromIntegral tn) (fromIntegral n)
+
+          e -> throwIO $ BadFormException @C (mkList e)
+
+        entry $ bindMatch "test:ncq2:simple1" $ nil_ $ const $ do
+            runTest testNCQ2Simple1
+
+        entry $ bindMatch "test:ncq2:repair1" $ nil_ $ const $ do
+            runTest testNCQ2Repair1
+
+        entry $ bindMatch "test:ncq2:filefastcheck" $ nil_ $ \case
+          [ StringLike fn ] -> do
+            ncqFileFastCheck fn
+
+          e -> throwIO $ BadFormException @C (mkList e)
+
+        entry $ bindMatch "test:ncq2:concurrent:write:simple1" $ nil_ $ \case
+          [ LitIntVal tn, LitIntVal n ] -> do
+            runTest $ testNCQ2ConcurrentWriteSimple1 ( fromIntegral tn) (fromIntegral n)
+
+          e -> throwIO $ BadFormException @C (mkList e)
+
 
         entry $ bindMatch "test:filter:emulate-1" $ nil_ $ \case
           [ LitIntVal n ] -> runTest $ testFilterEmulate1 (fromIntegral n)