diff --git a/Remote/Directory.hs b/Remote/Directory.hs
index 006638a2f2..6bf725379c 100644
--- a/Remote/Directory.hs
+++ b/Remote/Directory.hs
@@ -19,8 +19,8 @@ import Config
 import Utility.FileMode
 import Remote.Helper.Special
 import Remote.Helper.Encryptable
+import Remote.Helper.Chunked
 import Crypto
-import Utility.DataUnits
 import Data.Int
 import Annex.Content
 
@@ -58,19 +58,6 @@ gen r u c = do
 			remotetype = remote
 		}
 
-type ChunkSize = Maybe Int64
-
-chunkSize :: Maybe RemoteConfig -> ChunkSize
-chunkSize Nothing = Nothing
-chunkSize (Just m) =
-	case M.lookup "chunksize" m of
-		Nothing -> Nothing
-		Just v -> case readSize dataUnits v of
-			Nothing -> error "bad chunksize"
-			Just size
-				| size <= 0 -> error "bad chunksize"
-				| otherwise -> Just $ fromInteger size
-
 directorySetup :: UUID -> RemoteConfig -> Annex RemoteConfig
 directorySetup u c = do
 	-- verify configuration is sane
@@ -89,14 +76,6 @@ directorySetup u c = do
 locations :: FilePath -> Key -> [FilePath]
 locations d k = map (d </>) (keyPaths k)
 
-{- An infinite stream of chunks to use for a given file. -}
-chunkStream :: FilePath -> [FilePath]
-chunkStream f = map (\n -> f ++ ".chunk" ++ show n) [1 :: Integer ..]
-
-{- A file that records the number of chunks used. -}
-chunkCount :: FilePath -> FilePath
-chunkCount f = f ++ ".chunkcount"
-
 withCheckedFiles :: (FilePath -> IO Bool) -> ChunkSize -> FilePath -> Key -> ([FilePath] -> IO Bool) -> IO Bool
 withCheckedFiles _ _ [] _ _ = return False
 withCheckedFiles check Nothing d k a = go $ locations d k
@@ -107,18 +86,14 @@ withCheckedFiles check (Just _) d k a = go $ locations d k
   where
 	go [] = return False
 	go (f:fs) = do
-		let chunkcount = chunkCount f
+		let chunkcount = f ++ chunkCount
 		ifM (check chunkcount)
 			( do
-				count <- readcount chunkcount
-				let chunks = take count $ chunkStream f
+				chunks <- getChunks f <$> readFile chunkcount
 				ifM (all id <$> mapM check chunks)
 					( a chunks , return False )
 			, go fs
 			)
-	readcount f = fromMaybe (error $ "cannot parse " ++ f)
-		. (readish :: String -> Maybe Int)
-		<$> readFile f
 
 withStoredFiles :: ChunkSize -> FilePath -> Key -> ([FilePath] -> IO Bool) -> IO Bool
 withStoredFiles = withCheckedFiles doesFileExist
@@ -203,45 +178,26 @@ meteredWriteFile' meterupdate dest startstate feeder =
 		meterupdate $ toInteger $ S.length c
 		feed state cs h
 
-{- Generates a list of destinations to write to in order to store a key.
- - When chunksize is specified, this list will be a list of chunks.
- - The action should store the file, and return a list of the destinations
- - it stored it to, or [] on error.
- - The stored files are only put into their final place once storage is
- - complete.
- -}
 storeHelper :: FilePath -> ChunkSize -> Key -> ([FilePath] -> IO [FilePath]) -> Annex Bool
-storeHelper d chunksize key a = prep <&&> check <&&> go
+storeHelper d chunksize key storer = check <&&> go
   where
-	desttemplate = Prelude.head $ locations d key
-	dir = parentDir desttemplate
-	tmpdests = case chunksize of
-		Nothing -> [desttemplate ++ tmpprefix]
-		Just _ -> map (++ tmpprefix) (chunkStream desttemplate)
-	tmpprefix = ".tmp"
-	detmpprefix f = take (length f - tmpprefixlen) f
-	tmpprefixlen = length tmpprefix
-	prep = liftIO $ catchBoolIO $ do
-		createDirectoryIfMissing True dir
-		allowWrite dir
-		return True
+	basedest = Prelude.head $ locations d key
+	dir = parentDir basedest
 	{- The size is not exactly known when encrypting the key;
 	 - this assumes that at least the size of the key is
 	 - needed as free space. -}
 	check = checkDiskSpace (Just dir) key 0
 	go = liftIO $ catchBoolIO $ do
-		stored <- a tmpdests
-		forM_ stored $ \f -> do
-			let dest = detmpprefix f
-			renameFile f dest
-			preventWrite dest
-		when (chunksize /= Nothing) $ do
-			let chunkcount = chunkCount desttemplate
-			_ <- tryIO $ allowWrite chunkcount
-			writeFile chunkcount (show $ length stored)
-			preventWrite chunkcount
-		preventWrite dir
-		return (not $ null stored)
+		createDirectoryIfMissing True dir
+		allowWrite dir
+		preventWrite dir `after` storeChunks basedest chunksize storer recorder finalizer
+	finalizer f dest = do
+		renameFile f dest
+		preventWrite dest
+	recorder f s = do
+		void $ tryIO $ allowWrite f
+		writeFile f s
+		preventWrite f
 
 retrieve :: FilePath -> ChunkSize -> Key -> AssociatedFile -> FilePath -> Annex Bool
 retrieve d chunksize k _ f = metered Nothing k $ \meterupdate ->
diff --git a/Remote/Helper/Chunked.hs b/Remote/Helper/Chunked.hs
new file mode 100644
index 0000000000..59117eaca3
--- /dev/null
+++ b/Remote/Helper/Chunked.hs
@@ -0,0 +1,121 @@
+{- git-annex chunked remotes
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Remote.Helper.Chunked where
+
+import Common.Annex
+import Utility.DataUnits
+import Types.Remote
+
+import qualified Data.Map as M
+import qualified Data.ByteString.Lazy as L
+import Data.Int
+import qualified Control.Exception as E
+
+type ChunkSize = Maybe Int64
+
+{- Gets a remote's configured chunk size. -}
+chunkSize :: Maybe RemoteConfig -> ChunkSize
+chunkSize Nothing = Nothing
+chunkSize (Just m) =
+	case M.lookup "chunksize" m of
+		Nothing -> Nothing
+		Just v -> case readSize dataUnits v of
+			Nothing -> error "bad chunksize"
+			Just size
+				| size <= 0 -> error "bad chunksize"
+				| otherwise -> Just $ fromInteger size
+
+{- This is an extension that's added to the usual file (or whatever)
+ - where the remote stores a key. -}
+type ChunkExt = String
+
+{- A record of the number of chunks used.
+ -
+ - While this can be guessed at based on the size of the key, encryption
+ - makes that larger. Also, using this helps deal with changes to chunksize
+ - over the life of a remote.
+ -}
+chunkCount :: ChunkExt
+chunkCount = ".chunkcount"
+
+{- Parses the String from the chunkCount file, and returns the files that
+ - are used to store the chunks. -}
+getChunks :: FilePath -> String -> [FilePath]
+getChunks basedest chunkcount = take count $ map (basedest ++) chunkStream
+  where
+	count = fromMaybe 0 $ readish chunkcount
+
+{- An infinite stream of extensions to use for chunks. -}
+chunkStream :: [ChunkExt]
+chunkStream = map (\n -> ".chunk" ++ show n) [1 :: Integer ..]
+
+{- Given the base destination to use to store a value,
+ - generates a stream of temporary destinations (just one when not chunking)
+ - and passes it to an action, which should chunk and store the data,
+ - and return the destinations it stored to, or [] on error.
+ -
+ - Then calles the finalizer to rename the temporary destinations into
+ - their final places (and do any other cleanup), and writes the chunk count
+ - (if chunking)
+ -}
+storeChunks :: FilePath -> ChunkSize -> ([FilePath] -> IO [FilePath]) -> (FilePath -> String -> IO ()) -> (FilePath -> FilePath -> IO ()) -> IO Bool
+storeChunks basedest chunksize storer recorder finalizer =
+	either (const $ return False) return
+		=<< (E.try go :: IO (Either E.SomeException Bool))
+  where
+	go = do
+		stored <- storer tmpdests
+		forM_ stored $ \d -> do
+			let dest = detmpprefix d
+			finalizer d dest
+		when (chunksize /= Nothing) $ do
+			let chunkcount = basedest ++ chunkCount
+			recorder chunkcount (show $ length stored)
+		return (not $ null stored)
+
+	tmpprefix = ".tmp"
+	detmpprefix f = take (length f - tmpprefixlen) f
+	tmpprefixlen = length tmpprefix
+	tmpdests
+		| chunksize == Nothing = [basedest ++ tmpprefix]
+		| otherwise = map (++ tmpprefix) $ map (basedest ++) chunkStream
+
+{- Given a list of destinations to use, chunks the data according to the
+ - ChunkSize, and runs the storer action to store each chunk. Returns
+ - the destinations where data was stored, or [] on error.
+ -
+ - This buffers each chunk in memory.
+ - More optimal versions of this can be written, that rely
+ - on L.toChunks to split the lazy bytestring into chunks (typically
+ - smaller than the ChunkSize), and eg, write those chunks to a Handle.
+ - But this is the best that can be done with the storer interface that
+ - writes a whole L.ByteString at a time.
+ -}
+storeChunked :: ChunkSize -> [FilePath] -> (FilePath -> L.ByteString -> IO ()) -> L.ByteString -> IO [FilePath]
+storeChunked chunksize dests storer content =
+	either (const $ return []) return
+		=<< (E.try (go chunksize dests) :: IO (Either E.SomeException [FilePath]))
+  where
+	go _ [] = return [] -- no dests!?
+
+	go Nothing (d:_) = do
+		storer d content
+		return [d]
+
+	go (Just sz) _
+		-- always write a chunk, even if the data is 0 bytes
+		| L.null content = go Nothing dests
+		| otherwise = storechunks sz [] dests content
+		
+	storechunks _ _ [] _ = return [] -- ran out of dests
+	storechunks sz useddests (d:ds) b
+		| L.null b = return $ reverse useddests
+		| otherwise = do
+			let (chunk, b') = L.splitAt sz b
+			storer d chunk
+			storechunks sz (d:useddests) ds b'