annex.fastcopy

Added annex.fastcopy and remote.name.annex-fastcopy config setting. When
set, this allows the copy_file_range syscall to be used, which can eg allow
for server-side copies on NFS. (For fastest copying, also disable
annex.verify or remote.name.annex-verify.)

This is a simple implementation, that does not handle resuming as well as
it possibly could.

It can be used with both local git remotes (including on NFS), and
directory special remotes. Other types of remotes could in theory also
support it, so I've left the config documented as a general thing.

Utility/CopyFile.hs

@@ -43,7 +43,12 @@ copyMetaDataParams meta = map snd $ filter fst
 
 {- The cp command is used, because I hate reinventing the wheel,
  - and because this allows easy access to features like cp --reflink
- - and preserving metadata. -}
+ - and preserving metadata.
+ -
+ - This uses --reflink=auto when supported, which allows for fast copies
+ - using reflinks or the copy_file_range syscall. Whatever cp thinks is
+ - best. --reflink=auto is the default of recent versions of cp, but is
+ - used explicitly to support older versions. -}
 copyFileExternal :: CopyMetaData -> OsPath -> OsPath -> IO Bool
 copyFileExternal meta src dest = do
 	-- Delete any existing dest file because an unwritable file
@@ -81,8 +86,8 @@ copyCoW meta src dest
 	| otherwise = return False
   where
  	-- Note that in coreutils 9.0, cp uses CoW by default,
-	-- without needing an option. This s only needed to support 
-	-- older versions.
+	-- without needing an option. But, this makes it fail if it is
+	-- unable to make a CoW copy.
 	params = Param "--reflink=always" : copyMetaDataParams meta
 
 {- Create a hard link if the filesystem allows it, and fall back to copying