Re: [PATCH v1 0/8] VFS: In-kernel copy system call

On 09/08/2015 11:21 AM, Pádraig Brady wrote:

On 04/09/15 21:16, Anna Schumaker wrote:

quoted

Copy system calls came up during Plumbers a couple of weeks ago, because
several filesystems (including NFS and XFS) are currently working on copy
acceleration implementations.  We haven't heard from Zach Brown in a while,
so I volunteered to push his patches upstream so individual filesystems
don't need to keep writing their own ioctls.

Just mentioning that this is just pertaining to the data, not the metadata.
Providing metadata copying facilities would be _very_ useful, as
most file system specific details relate to the metadata, and having
VFS operations for that would avoid the plethora of details in each userspace tool,
and theoretically support translations between disparate metadata.

Metadata copying could be added later if somebody really wants it.

quoted

The first three patches are a simple reposting of Zach's patches from several
months ago, with one minor error code fix.  The remaining patches add in a
fallback mechanism when filesystems don't provide a copy function.  This is
especially useful when doing a server-side copy on NFS (using the new COPY
operation in NFS v4.2).  This fallback can be disabled by passing the flag
COPY_REFLINK to the system call.

I see copy_file_range() is a reflink() on BTRFS?
That's a bit surprising, as it avoids the copy completely.
cp(1) for example considered doing a BTRFS clone by default,
but didn't due to expectations that users actually wanted
the data duplicated on disk for resilience reasons,
and for performance reasons so that write latencies were
restricted to the copy operation, rather than being
introduced at usage time as the dest file is CoW'd.

If reflink() is a possibility for copy_file_range()
then could it be done optionally with a flag?

The idea is that filesystems get to choose how to handle copies in the default case.  BTRFS could do a reflink, but NFS could do a server side copy instead.  I can change the default behavior to only do a data copy (unless the reflink flag is specified) instead, if that is desirable.

What does everybody think?

Anna

thanks,
Pádraig