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

[Omar Sandoval <osandov@osandov.com>]

On Mon, Aug 22, 2016 at 08:43:18PM -0600, Chris Murphy wrote:
> On Mon, Aug 22, 2016 at 5:06 PM, Darrick J. Wong
> <darrick.wong@oracle.com> wrote:
> > [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.
> 
> Use case wise I can't think of why I'd want to do unshare. There is a
> use case for wanting to set nocow after the fact. I have no idea what
> complexity is added on the Btrfs side for either operation, it seems
> like at the least to set it, data csum needs a way to be ignored or
> removed; and conversely to unset nocow it's a question whether that
> means the file should have csum's computed, strictly speaking I guess
> you could have cow without datacsum.

One use case is for swapfile support on Btrfs -- I implemented it with
the requirement that the file was nocow with no shared extents. I think
there was some discussion about having the swapon operation do that
unshare, but I decided against that [1]. (I should take a look at
reviving that patch series.)

Darrick, what's XFS doing for reflink + swap files?

1: http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg785536.html

-- 
Omar