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    SubjectRe: what is necessary for directory hard links
    On 7/24/06, <> wrote:
    > On Mon, 24 Jul 2006 10:45:45 +0400, Nikita Danilov said:
    > > Joshua Hudson writes:
    > > > In my filesystem, any attempt to create a loop of hard links
    > > > is detected and cancelled.
    > >
    > > Can you elaborate a bit on this exciting mechanism? Obviously an ability
    > > to efficiently detect loops would be a break-through in a
    > > reference-counted garbage collection, somehow missed for last 40
    > It's actually pretty trivial to do if it's a toy filesystem and all the
    > relevant inodes are in-memory already. The hard-to-solve part is getting
    > around the (apparent) need to walk across essentially the entire tree
    > structure making sure that you aren't creating a loop. This can get
    > rather performance piggy - even /home on my laptop has some 400K
    > inodes on it, and a 'find /home -type d' takes 28 seconds. That's a *long*
    > time to lock and freeze a filesystem.

    Actually, I walk from the source inode down to try to find the
    target inode. If not found, this is not attempting to create a loop.
    Should be obvious that the average case is much less than the
    whole tree.

    > Where it gets *really* messy is that it isn't just mkdir that's the problem -
    > once you let there be more than one path from the fs root to a given directory,
    > it gets *really* hard to make sure that any given 'mv' command isn't going to
    > to screw things up (is 'mv a/b/c/d ../../w/z/b' safe? How do you know, without
    > examining a *lot* of stuff under a/ and ../../w/?

    mv /a/b/c/d ../../w/z/b is implemented as this in the filesystem:
    ln /a/b/c/d ../../w/z/b && rm /a/b/c/d

    So what it's going to do is try to find z under /a/b/c/d.

    Oh, and Nakita's right about the NFS server stuff. Actually, I think
    the current filesystem I use for this is totally incompatible with
    NFS (cannot call d_splice_alias on directory dnodes) so that
    doesn't concern me.
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      Last update: 2006-07-24 18:23    [W:0.024 / U:60.744 seconds]
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