diff --git a/src/journal/src/Journal/Internal/ByteBufferPtr.hs b/src/journal/src/Journal/Internal/ByteBufferPtr.hs
new file mode 100644
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+++ b/src/journal/src/Journal/Internal/ByteBufferPtr.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE UnboxedTuples #-}
+
+module Journal.Internal.ByteBufferPtr where
+
+import Control.Exception
+import Control.Monad
+import Data.Bits
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Internal as BS
+import qualified Data.ByteString.Lazy as LBS
+import qualified Data.ByteString.Lazy.Internal as LBS
+import Data.IORef
+import Data.Int
+import Data.Word
+import Foreign (withForeignPtr)
+import Foreign.Concurrent
+import Foreign.Storable
+import GHC.Exts
+import GHC.ForeignPtr
+import GHC.IO
+import GHC.Stack
+import GHC.Types
+import System.Posix.IO
+       (OpenMode(ReadWrite), closeFd, defaultFileFlags, openFd)
+
+import Journal.Internal.Mmap
+
+------------------------------------------------------------------------
+-- * Types
+
+data ByteBuffer = ByteBuffer
+  { bbData     :: {-# UNPACK #-} !(ForeignPtr Word8)
+  , bbCapacity :: {-# UNPACK #-} !Capacity
+  , bbLimit    :: {-# UNPACK #-} !(IORef Limit)
+  , bbPosition :: {-# UNPACK #-} !(IORef Position)
+  , bbMark     :: {-# UNPACK #-} !(IORef Position)
+  , bbSlice    :: {-# UNPACK #-} !(IORef Slice)
+  -- XXX: ByteOrder / Endianess?
+  }
+
+newtype Capacity = Capacity { unCapacity :: Int }
+  deriving (Num, Integral, Real, Ord, Eq, Enum)
+
+newtype Limit = Limit Int
+  deriving (Num, Integral, Real, Ord, Eq, Enum)
+
+newtype Position = Position { unPosition :: Int }
+  deriving (Num, Integral, Real, Ord, Eq, Enum)
+
+newtype Slice = Slice Int
+  deriving (Num, Integral, Real, Ord, Eq, Enum)
+
+------------------------------------------------------------------------
+
+newByteBuffer :: ForeignPtr Word8 -> Capacity -> Limit -> Position -> Maybe (IORef Slice)
+              -> IO ByteBuffer
+newByteBuffer fptr capa lim pos mSli
+  = ByteBuffer fptr capa
+  <$> newIORef lim
+  <*> newIORef pos
+  <*> newIORef (-1)
+  <*> maybe (newIORef 0) return mSli
+
+bbPtr :: ByteBuffer -> ForeignPtr Word8
+bbPtr (ByteBuffer fptr _ _ _ _ _) = fptr
+{-# INLINE bbPtr #-}
+
+getCapacity :: ByteBuffer -> Capacity
+getCapacity = bbCapacity
+{-# INLINE getCapacity #-}
+
+readLimit :: ByteBuffer -> IO Limit
+readLimit = readIORef . bbLimit
+{-# INLINE readLimit #-}
+
+writeLimit :: ByteBuffer -> Limit -> IO ()
+writeLimit bb = writeIORef (bbLimit bb)
+{-# INLINE writeLimit #-}
+
+readPosition :: ByteBuffer -> IO Position
+readPosition = readIORef . bbPosition
+{-# INLINE readPosition #-}
+
+writePosition :: ByteBuffer -> Position -> IO ()
+writePosition bb = writeIORef (bbPosition bb)
+{-# INLINE writePosition #-}
+
+incrPosition :: ByteBuffer -> Int -> IO ()
+incrPosition bb i = modifyIORef (bbPosition bb) (+ fromIntegral i)
+{-# INLINE incrPosition #-}
+
+readMark :: ByteBuffer -> IO Position
+readMark = readIORef . bbMark
+{-# INLINE readMark #-}
+
+writeMark :: ByteBuffer -> Position -> IO ()
+writeMark bb = writeIORef (bbMark bb)
+{-# INLINE writeMark #-}
+
+readSlice :: ByteBuffer -> IO Slice
+readSlice = readIORef . bbSlice
+
+writeSlice :: ByteBuffer -> Slice -> IO ()
+writeSlice bb = writeIORef (bbSlice bb)
+
+------------------------------------------------------------------------
+
+remaining :: ByteBuffer -> IO Int
+remaining bb = do
+  lim <- readLimit bb
+  pos <- readPosition bb
+  return (fromIntegral (lim - fromIntegral pos))
+
+------------------------------------------------------------------------
+-- * Checks
+
+boundCheck :: HasCallStack => ByteBuffer -> Int -> IO ()
+boundCheck bb ix = do
+  -- XXX: ix + slice?
+  -- XXX: parametrise on build flag and only do these checks if enabled?
+  if ix < fromIntegral (getCapacity bb)
+  then return ()
+  else do
+    putStrLn (prettyCallStack callStack)
+    throwIO (IndexOutOfBounds errMsg)
+  where
+    errMsg = concat
+      [ "boundCheck: index out of bounds "
+      , "(", show ix, ",", show (unCapacity (getCapacity bb)), ")"
+      ]
+
+invariant :: ByteBuffer -> IO ()
+invariant bb = do
+  mark <- readMark bb
+  pos  <- readPosition bb
+  lim  <- readLimit bb
+  let capa = getCapacity bb
+  assert ((mark == (-1) || 0 <= mark) &&
+          mark <= fromIntegral pos &&
+          pos <= fromIntegral lim &&
+          lim <= fromIntegral capa)
+    (return ())
+
+------------------------------------------------------------------------
+-- * Create
+
+-- allocate :: Int -> IO ByteBuffer
+-- allocate capa@(I# capa#) = IO $ \s ->
+--   case newPinnedByteArray# capa# s of
+--     (# s', mba# #) -> unIO (newByteBuffer mba# (Capacity capa) (Limit capa) 0 Nothing) s'
+
+allocateAligned :: Int -> Int -> IO ByteBuffer
+allocateAligned size align = do
+  fptr <- posixMemalignFPtr size align
+  newByteBuffer fptr (Capacity size) (Limit size) 0 Nothing
+
+mmapped :: FilePath -> Int -> IO ByteBuffer
+mmapped fp capa = do
+  fd <- openFd fp ReadWrite Nothing defaultFileFlags
+  pageSize <- sysconfPageSize
+  ptr <- mmap Nothing (fromIntegral capa)
+           (pROT_READ .|. pROT_WRITE) mAP_SHARED (Just fd) 0
+  closeFd fd
+  fptr <- newForeignPtr ptr (finalizer ptr pageSize)
+  newByteBuffer fptr (Capacity pageSize) (Limit pageSize) 0 Nothing
+  where
+    finalizer :: Ptr a -> Int -> IO ()
+    finalizer ptr size = munmap ptr (fromIntegral size)
+
+wrap :: ByteBuffer -> IO ByteBuffer
+wrap bb = newByteBuffer (bbData bb) capa lim (Position 0) (Just (bbSlice bb))
+  where
+    capa = bbCapacity bb
+    lim  = Limit (fromIntegral capa)
+
+wrapPart :: ByteBuffer -> Int -> Int -> IO ByteBuffer
+wrapPart bb offset len = newByteBuffer (bbData bb) capa lim pos (Just (bbSlice bb))
+  where
+    capa = Capacity len
+    lim  = Limit (fromIntegral offset + fromIntegral len)
+    pos  = Position (fromIntegral offset)
+
+slice :: ByteBuffer -> IO ByteBuffer
+slice bb = do
+  pos <- readPosition bb
+  left <- remaining bb
+  slice <- newIORef (fromIntegral pos)
+  newByteBuffer (bbData bb) (Capacity left) (Limit left) (Position 0) (Just slice)
+
+duplicate :: ByteBuffer -> IO ByteBuffer
+duplicate bb = do
+  lim <- readLimit bb
+  pos <- readPosition bb
+  newByteBuffer (bbData bb) (getCapacity bb) lim pos (Just (bbSlice bb))
+
+------------------------------------------------------------------------
+
+mark :: ByteBuffer -> IO ()
+mark bb = do
+  pos <- readPosition bb
+  writeMark bb pos
+
+compact :: ByteBuffer -> IO ByteBuffer
+compact = undefined
+
+------------------------------------------------------------------------
+
+-- | Clears the byte buffer. The position is set to zero, the limit is set to
+-- the capacity, and the mark is discarded.
+clear :: ByteBuffer -> IO ()
+clear bb = do
+  writePosition bb 0
+  let Capacity capa = getCapacity bb
+  writeLimit bb (Limit capa)
+  writeMark bb (-1)
+
+  {-
+clean :: ByteBuffer -> IO ()
+clean bb = do
+  Position (I# from#) <- readPosition bb
+  Limit (I# to#) <- readLimit bb
+  IO $ \s -> case setByteArray# (bbData bb) from# (to# -# from#) 0# s of
+    s' -> (# s', () #)
+-}
+
+-- | Flips the byte buffer. The limit is set to the current position and then
+-- the position is set to zero. If the mark is defined then it is discarded.
+flipBB :: ByteBuffer -> IO ()
+flipBB bb = do
+  Position pos <- readPosition bb
+  writeLimit bb (Limit pos)
+  writePosition bb 0
+  writeMark bb (-1)
+
+-- | Rewinds the byte buffer. The position is set to zero and the mark is
+-- discarded.
+rewind :: ByteBuffer -> IO ()
+rewind bb = do
+  writePosition bb 0
+  writeMark bb (-1)
+
+-- | Resets the byte buffer's position to the previously marked position.
+reset :: ByteBuffer -> IO ()
+reset bb = do
+  mrk <- readMark bb
+  writePosition bb mrk
+
+------------------------------------------------------------------------
+-- * Single-byte relative and absolute operations
+
+putByte :: ByteBuffer -> Word8 -> IO ()
+putByte = undefined
+
+getByte :: ByteBuffer -> IO Word8
+getByte bb = do
+  pos <- readPosition bb
+  w8 <- indexWord8OffAddr bb (unPosition pos)
+  writePosition bb (pos + 1)
+  return w8
+
+putByteAt :: ByteBuffer -> Int -> Word8 -> IO ()
+putByteAt = undefined
+
+getByteAt :: ByteBuffer -> Int -> IO Word8
+getByteAt = undefined
+
+------------------------------------------------------------------------
+-- * Multi-byte relative and absolute operations
+
+  {-
+putBytes :: ByteBuffer -> ByteBuffer -> IO ()
+putBytes src dest = do
+  Position (I# destPos#) <- readPosition dest
+  let Capacity srcCapa@(I# srcCapa#) = getCapacity src
+  -- XXX: bounds check
+  IO $ \s ->
+    case copyMutableByteArray# (bbData src) 0# (bbData dest) destPos# srcCapa# s of
+      s' -> (# s', () #)
+  incrPosition dest srcCapa
+
+getBytes :: ByteBuffer -> Int -> Int -> IO [Word8]
+getBytes bb offset len = undefined
+
+putByteString :: ByteBuffer -> BS.ByteString -> IO ()
+putByteString bb bs = do
+  let (fptr, I# offset#, I# len#) = BS.toForeignPtr bs
+  boundCheck bb (I# (len# -# 1#))
+  withForeignPtr fptr $ \(Ptr addr#) -> IO $ \s ->
+    case copyAddrToByteArray# addr# (bbData bb) offset# len# s of
+      s' -> (# s', () #)
+
+putLazyByteString :: ByteBuffer -> LBS.ByteString -> IO ()
+putLazyByteString bb lbs = do
+  let (fptr, I# offset#, I# len#) = BS.toForeignPtr (LBS.toStrict lbs)
+  boundCheck bb (I# (len# -# 1#))
+  withForeignPtr fptr $ \(Ptr addr#) -> IO $ \s ->
+    case copyAddrToByteArray# addr# (bbData bb) offset# len# s of
+      s' -> (# s', () #)
+
+getByteString :: ByteBuffer -> Int -> IO BS.ByteString
+getByteString bb len@(I# len#) = do
+  boundCheck bb (len - 1)
+  Position (I# offset#) <- readPosition bb
+  bs <- BS.create len $ \(Ptr addr#) -> IO $ \s ->
+    case copyMutableByteArrayToAddr# (bbData bb) offset# addr# len# s of
+      s' -> (# s', () #)
+  incrPosition bb len
+  return bs
+
+getLazyByteString :: ByteBuffer -> Int -> IO LBS.ByteString
+getLazyByteString bb len = do
+  bs <- getByteString bb len
+  return (LBS.fromStrict bs)
+
+getByteStringAt :: ByteBuffer -> Int -> Int -> IO BS.ByteString
+getByteStringAt bb offset len = do
+  undefined
+-}
+
+------------------------------------------------------------------------
+-- * Relative operations on `Storable` elements
+
+putStorable :: Storable a => ByteBuffer -> a -> IO ()
+putStorable bb x = do
+  pos <- readPosition bb
+  putStorableAt bb (fromIntegral pos) x
+  incrPosition bb (sizeOf x)
+
+getStorable :: Storable a => ByteBuffer -> IO a
+getStorable bb = do
+  pos <- readPosition bb
+  x <- getStorableAt bb (fromIntegral pos)
+  incrPosition bb (sizeOf x)
+  return x
+
+------------------------------------------------------------------------
+-- * Absolute operations on `Storable` elements
+
+putStorableAt :: Storable a => ByteBuffer -> Int -> a -> IO ()
+putStorableAt bb ix x = do
+  boundCheck bb ix
+  withForeignPtr (bbPtr bb) $ \ptr ->
+    pokeByteOff ptr ix x
+
+getStorableAt :: Storable a => ByteBuffer -> Int -> IO a
+getStorableAt bb ix = do
+  boundCheck bb ix
+  withForeignPtr (bbPtr bb) $ \ptr ->
+    peekByteOff ptr ix
+
+------------------------------------------------------------------------
+
+{-
+
+-- readCharOffArray#
+-- readWideCharOffArray#
+readIntOffArrayIx :: ByteBuffer -> Int -> IO Int
+readIntOffArrayIx bb ix@(I# ix#) = do
+  boundCheck bb ix
+  IO $ \s ->
+    case readIntArray# (bbData bb) ix# s of
+      (# s', i #) -> (# s', I# i #)
+-- readWordOffArray#
+-- readArrayOffAddr#
+-- readFloatOffArray#
+-- readDoubleOffArray#
+-- readStablePtrOffArray#
+-- readInt8OffArray#
+-- readInt16OffArray#
+readInt32OffArrayIx :: ByteBuffer -> Int -> IO Int32
+readInt32OffArrayIx bb ix@(I# ix#) = do
+  boundCheck bb ix
+  IO $ \s ->
+    case readInt32Array# (bbData bb) ix# s of
+      (# s', i #) -> (# s', fromIntegral (I# i) #)
+
+readInt64OffArrayIx :: ByteBuffer -> Int -> IO Int64
+readInt64OffArrayIx bb ix@(I# ix#) = do
+  boundCheck bb ix
+  IO $ \s ->
+    case readInt64Array# (bbData bb) ix# s of
+      (# s', i #) -> (# s', fromIntegral (I# i) #)
+-}
+
+indexWord8OffAddr :: ByteBuffer -> Int -> IO Word8
+indexWord8OffAddr bb offset@(I# offset#) = do
+  boundCheck bb offset
+  withForeignPtr (bbPtr bb) $ \(Ptr addr#) ->
+    return (fromIntegral (W# (indexWord8OffAddr# addr# offset#)))
+
+  {-
+-- readWord16OffArray#
+-- readWord32OffArray#
+-- readWord64OffArray#
+
+-- writeCharOffArray#
+-- writeWideCharOffArray#
+writeInt = writeIntOffArrayIx
+
+writeIntOffArrayIx :: ByteBuffer -> Int -> Int -> IO ()
+writeIntOffArrayIx bb ix@(I# ix#) (I# value#) = do
+  boundCheck bb ix
+  IO $ \s ->
+    case writeIntArray# (bbData bb) ix# value# s of
+      s' -> (# s', () #)
+-- writeWordOffArray#
+-- writeArrayOffAddr#
+-- writeFloatOffArray#
+-- writeDoubleOffArray#
+-- writeStablePtrOffArray#
+-- writeInt8OffArray#
+-- writeInt16OffArray#
+
+writeInt32OffArrayIx :: ByteBuffer -> Int -> Int32 -> IO ()
+writeInt32OffArrayIx bb ix@(I# ix#) value = do
+  boundCheck bb ix
+  IO $ \s ->
+    case writeInt32Array# (bbData bb) ix# value# s of
+      s' -> (# s', () #)
+  where
+    I# value# = fromIntegral value
+
+writeInt64OffArrayIx :: ByteBuffer -> Int -> Int64 -> IO ()
+writeInt64OffArrayIx bb ix@(I# ix#) value = do
+  boundCheck bb ix
+  IO $ \s ->
+    case writeInt64Array# (bbData bb) ix# value# s of
+      s' -> (# s', () #)
+  where
+    I# value# = fromIntegral value
+
+-- writeWord8OffArray#
+-- writeWord16OffArray#
+writeWord32OffArrayIx :: ByteBuffer -> Int -> Word32 -> IO ()
+writeWord32OffArrayIx bb offset@(I# offset#) value = do
+  boundCheck bb offset
+  IO $ \s ->
+    case writeWord32Array# (bbData bb) offset# value# s of
+      s' -> (# s', () #)
+  where
+    W# value# = fromIntegral value
+-- writeWord64OffArray#
+
+-- atomicReadIntArray#
+-- atomicWriteIntArray#
+
+-- | Given a bytebuffer, an offset in machine words, the expected old value, and
+-- the new value, perform an atomic compare and swap i.e. write the new value if
+-- the current value matches the provided old value. Returns a boolean
+-- indicating whether the compare and swap succeded or not. Implies a full
+-- memory barrier.
+casIntArray :: ByteBuffer -> Int -> Int -> Int -> IO Bool
+casIntArray bb offset@(I# offset#) (I# old#) (I# new#) = do
+  boundCheck bb offset
+  IO $ \s ->
+    case casIntArray# (bbData bb) offset# old# new# s of
+      (# s', before# #) -> case before# ==# old# of
+        1# -> (# s', True #)
+        0# -> (# s', False #)
+
+-- | Given a bytebuffer, and offset in machine words, and a value to add,
+-- atomically add the value to the element. Returns the value of the element
+-- before the operation. Implies a full memory barrier.
+fetchAddIntArray :: ByteBuffer -> Int -> Int -> IO Int
+fetchAddIntArray bb offset@(I# offset#) (I# incr#) = do
+  boundCheck bb offset
+  IO $ \s ->
+    case fetchAddIntArray# (bbData bb) offset# incr# s of
+      (# s', before# #) -> (# s', I# before# #)
+
+-- | Given a bytebuffer, and offset in machine words, and a value to add,
+-- atomically add the value to the element. Implies a full memory barrier.
+fetchAddIntArray_ :: ByteBuffer -> Int -> Int -> IO ()
+fetchAddIntArray_ bb offset@(I# offset#) (I# incr#) = do
+  boundCheck bb offset
+  IO $ \s ->
+    case fetchAddIntArray# (bbData bb) offset# incr# s of
+      (# s', _before# #) -> (# s', () #)
+
+-- | Given a bytebuffer, and offset in machine words, and a value to add,
+-- atomically add the value to the element. Returns the value of the element
+-- after the operation. Implies a full memory barrier.
+fetchAddWordAddr' :: ByteBuffer -> Int -> Word -> IO Int
+fetchAddWordAddr' bb offset@(I# offset#) (W# incr#) = do
+  boundCheck bb offset
+  IO $ \s ->
+    case fetchAddIntArray# (bbData bb) offset# incr# s of
+      (# s', before# #) -> (# s', I# (before# +# incr#) #)
+  -}
+
+------------------------------------------------------------------------
+-- * Mapped
+
+-- | Calls `msync` which forces the data in memory to be synced to disk.
+force :: ByteBuffer -> IO ()
+force bb =
+  withForeignPtr (bbPtr bb) $ \ ptr ->
+    msync ptr (fromIntegral (bbCapacity bb)) mS_SYNC False
+
+------------------------------------------------------------------------
+
+t2 :: IO ()
+t2 = do
+  ds <- replicateM 200000 $ do
+    bb <- mmapped "/tmp/mmap.txt" 4096
+    bb' <- duplicate bb
+    putStorable bb (0.1 :: Double)
+    putStorable bb 'A'
+    d <- getStorable bb'
+    return (d :: Double)
+  if all (== 0.1) ds
+  then putStrLn "success!"
+  else error "t2: bad"