copy_file_range and user space tools to do copy fastest

[Andreas Dilger]

On Apr 27, 2018, at 12:25 PM, Steve French <smfrench at gmail.com> 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.

Cheers, Andreas





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