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.w...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
