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    SubjectRe: [Ocfs2-devel] [GIT PULL] ocfs2 changes for 2.6.32
    On Fri, Sep 18, 2009 at 02:34:18PM +0100, Pádraig Brady wrote:
    > Joel Becker wrote:
    > > On Thu, Sep 17, 2009 at 09:29:14AM -0700, Linus Torvalds wrote:
    > >> Why would anybody want to hide it at all? Why even the libc hiding?
    > >>
    > >> Nobody is going to use this except for special apps. Let them see what
    > >> they can do, in all its glory.
    > >
    > > I expect everyone will use this through cp(1), so that cp(1) can
    > > try to get server-side copy on the network filesystms.
    > For reference, cp(1) has a --reflink option as of
    > coreutils-7.5 which currently just does:
    > ioctl (dest_fd, BTRFS_IOC_CLONE, src_fd);

    Note that the btrfs ioctl is not a reflink(), so this probably
    wants changing (OCFS2_IOC_REFLINK is the ocfs2 ioctl, sys_reflink() was
    going to be the syscall).

    > There is a specific option in cp to do this because
    > a "reflink copy" was seen to have these disadvantages:
    > 1. one copy of data blocks so more chances of data loss
    > 2. disk head seeking deferred to modification process
    > 3. possible fragmentation on write
    > 4. possible ENOSPC on write
    > Now 2. will go away with time, and 3 & 4 may be alleviated
    > by the use of fallocate(), but 1. was deemed important
    > enough to not enable by default.

    1, 2, and 3 are definitely in the category of "it would be nice
    to choose the behavior". 4 is the big one, because it breaks default
    cp(1) assumptions. The good news is that the current copyfile
    idea of copyfile(src, dst, 0) would satisfy 1-4 and be efficient or
    return -ENOTSUPP/-ENOSYS if it couldn't be. Then cp(1) falls back to
    the read-write loop.
    cp --reflink would become copyfile(src, dst, COPYFILE_SNAPSHOT)



    Life's Little Instruction Book #451

    "Don't be afraid to say, 'I'm sorry.'"

    Joel Becker
    Principal Software Developer
    Phone: (650) 506-8127
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