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    SubjectRe: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS
    Chris Mason wrote:
    > On Tue, Jun 19, 2007 at 10:11:13AM +0100, Pádraig Brady wrote:
    >>Vladislav Bolkhovitin wrote:
    >>>I would also suggest one more feature: support for block level
    >>>de-duplication. I mean:
    >>>1. Ability for Btrfs to have blocks in several files to point to the
    >>>same block on disk
    >>>2. Support for new syscall or IOCTL to de-duplicate as a single
    >>>transaction two or more blocks on disk, i.e. link them to one of them
    >>>and free others
    >>>3. De-de-duplicate blocks on disk, i.e. copy them on write
    >>>I suppose that de-duplication itself would be done by some user space
    >>>process that would scan files, determine blocks with the same data and
    >>>then de-duplicate them by using syscall or IOCTL (2).
    >>>That would be very usable feature, which in most cases would allow to
    >>>shrink occupied disk space on 50-90%.
    >>Have you references for this number?
    >>In my experience one gets a lot of benefit from
    >>the much simpler process of "de-duplication" of files.
    > Yes, I would expect simple hard links to be a better solution for this,
    > but the feature request is not that out of line.

    From effort POV hard links could be a better solution, but from
    effectiveness POV I can't agree with you.

    > I actually had plans
    > on implementing auto duplicate block reuse earlier in btrfs.
    > Snapshots already share duplicate blocks between files, and so all of
    > the reference counting needed to implement this already exists.
    > Snapshots are writable, and data mods are copy on write, and in general
    > things work.
    > But, to help fsck, the extent allocation tree has a back pointer to the
    > inode that owns an extent. If you're doing snapshots, all of the owners
    > of the extent have the same inode number. If you're sharing duplicate
    > blocks, the owners can have any inode number, and fsck becomes much more
    > complex.
    > In general, when I have to decide between fsck and a feature, I'm going
    > to pick fsck. The features are much more fun, but fsck is one of the
    > main motivations for doing this work.

    I see. Thanks for explaining your position.

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