Re: copy_file_range and user space tools to do copy fastest

[Eric Biggers]

On Fri, Apr 27, 2018 at 01:45:40PM -0600, Andreas Dilger wrote:
> On Apr 27, 2018, at 12:25 PM, Steve French <smfrench@xxxxxxxxx> wrote:
> > 
> > Are there any user space tools (other than our test tools and xfs_io
> > etc.) that support copy_file_range?  Looks like at least cp and rsync
> > and dd don't.  That syscall which now has been around a couple years,
> > and was reminded about at the LSF/MM summit a few days ago, presumably
> > is the 'best' way to copy a file fast since it tries all the
> > mechanisms (reflink etc.) in order.
> > 
> > Since copy_file_range syscall can be 100x or more faster for network
> > file systems than the alternative, was surprised when I noticed that
> > cp and rsync didn't support it.  It doesn't look like rsync even
> > supports reflink either(although presumably if you call
> > copy_file_range you don't have to worry about that), and reads/writes
> > are 8K. See copy_file() in rsync/util.c
> > 
> > In the cp command it looks like it can call the FICLONE IOCTL (see
> > clone_file() in coreutils/src/copy.c) but doesn't call the expected
> > "copy_file_range" syscall.
> > 
> > In the dd command it doesn't call either - see dd_copy in corutils/src/dd.c
> > 
> > Since it can be 100x or more faster in some cases to call
> > copy_file_range than do reads/writes back and forth to do a copy
> > (especially if network or clustered backend or cloud), what tools are
> > the best to recommend?
> > 
> > Would rsync or cp be likely to take patches to call the standard
> > "copy_file_range" syscall
> > (http://man7.org/linux/man-pages/man2/copy_file_range.2.html)?
> > Presumably not if it has been two+ years ... but would be interested
> > what copy tools to recommend to use instead.
> 
> I would start with submitting a patch to coreutils, if you can figure
> out that code enough to do so (I find it quite opaque).  Since it has
> been in the kernel for a while already, it should be acceptable to the
> upstream coreutils maintainers to use this interface.  Doubly so if you
> include some benchmarks with CIFS/NFS clients avoiding network overhead
> during the copy.
> 

For cp (coreutils), apparently there was a concern that copy_file_range()
expands holes; see the thread at
https://lists.gnu.org/archive/html/bug-coreutils/2016-09/msg00020.html.
Though, I'd think it could just be used on non-holes only.  And I don't think
the size_t type of 'len' is a problem either, since it's the copy length, not
the file size.  You just call it multiple times if the file is larger.

Eric