Re: Will Btrfs have an official command to "uncow" existing files?

["Darrick J. Wong" <darrick.wong@oracle.com>]

[add Dave and Christoph to cc]

On Mon, Aug 22, 2016 at 04:14:19PM -0400, Jeff Mahoney wrote:
> On 8/21/16 2:59 PM, Tomokhov Alexander wrote:
> > Btrfs wiki FAQ gives a link to example Python script: https://github.com/stsquad/scripts/blob/master/uncow.py
> > 
> > But such a crucial and fundamental tool must exist in stock btrfs-progs. Filesystem with CoW technology at it's core must provide user sufficient control over CoW aspects. Running 3rd-party or manually written scripts for filesystem properties/metadata manipulation is not convenient, not safe and definitely not the way it must be done.
> > 
> > Also is it possible (at least in theory) to "uncow" files being currently opened in-place? Without the trickery with creation & renaming of files or directories. So that running "chattr +C" on a file would be sufficient. If possible, is it going to be implemented?
> 
> XFS is looking to do this via fallocate using a flag that all file
> systems can choose to honor.  Once that lands, it would make sense for
> btrfs to use it as well.  The idea is that when you pass the flag in, we
> examine the range and CoW anything that has a refcount != 1.

There /was/ a flag to do that -- FALLOC_FL_UNSHARE_RANGE.  However,
Christoph and Dave felt[1] that the fallocate call didn't need to have
an explicit 'unshare' mode because unsharing shared blocks is
necessary to guarantee that a subsequent write will not ENOSPC.  I
felt that was sufficient justification to withdraw the unshare mode
flag.  If you fallocate the entire length of a shared file on XFS, it
will turn off CoW for that file until you reflink/dedupe it again.

At the time I wondered whether or not the btrfs developers (the list
was cc'd) would pipe up in support of the unshare flag, but nobody
did.  Consequently it remains nonexistent.  Christoph commented a few
months ago about unsharing fallocate over NFS atop XFS blocking for a
long time, though nobody asked for 'unshare' to be reinstated as a
separate fallocate mode, much less a 'don't unshare' flag for regular
fallocate mode.

(FWIW I'm ok with not having to fight for more VFS changes. :))

> That code hasn't landed yet though.  The last time I saw it posted was
> June.  I don't speak with knowledge of the integration plan, but it
> might just be queued up for the next merge window now that the reverse
> mapping patches have landed in 4.8.

I am going to try to land XFS reflink in 4.9; I hope to have an eighth
patchset out for review at the end of the week.

So... if the btrfs folks really want an unshare flag I can trivially
re-add it to the VFS headers and re-enable it in the XFS
implementation <cough> but y'all better speak up now and hammer out an
acceptable definition.  I don't think XFS needs a new flag.

--D

[1] https://www.spinics.net/lists/linux-nfs/msg54740.html

> 
> -Jeff
> 
> -- 
> Jeff Mahoney
> SUSE Labs
>