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    SubjectRe: [PATCH 3/3] btrfs: set FS_SUPPORTS_SEEK_HOLE flag.
    On Mon, 20 Apr 2015 02:48:55 -0700 Christoph Hellwig <hch@infradead.org>
    wrote:

    > On Mon, Apr 20, 2015 at 10:46:49AM +0100, David Howells wrote:
    > > NeilBrown <neilb@suse.de> wrote:
    > >
    > > > Missing patch 2 of the 3-patch series?
    > >
    > > Yes. :-)
    > >
    > > Do ext4 and xfs support this, do you know?
    >
    > Yes. As do f2fs, ocfs2, gfs2, ceph and NFSv4.2

    Are you sure about NFSv4.2?

    I see that it *can* report holes, but is there any guarantee that if you
    create a new file and write only the 5th block, then READ_PLUS will reliably
    report that the first 4 block are holes??

    Because if it doesn't guarantee that, then NFSv4.2 doesn't fit the with the
    others where SEEK_HOLE reliable reports holes.
    On the other hand if NFSv4.2 *does* guarantee that then the current READ_PLUS
    server patches are broken because they just use vfs_llseek and assume that
    trust what it says.

    It would be really nice if SEEK_{DATA,HOLE} either reported holes reliably or
    returned ENXIO, but I guess there was a goo reason not to do that.

    Thanks,
    NeilBrown
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