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    SubjectRe: [PATCH 00/32] VFS based Union Mount (V3)
    On Wed, May 20, 2009 at 11:05:27AM +0200, Miklos Szeredi wrote:
    > On Tue, 19 May 2009, Valerie Aurora wrote:
    > > As Jan said, readdir() of read-only unioned file systems works with a
    > > tmpfs top layer. If you think about it, this is the exact equivalent
    > > of the version of union mounts which used the in-kernel caching
    > > approach - except that it's better, because it reuses existing code
    > > and caches between readdir() calls. Cool, huh?
    > Yeah... OTOH tmpfs is probably a way too heavyweight solution for
    > cases where memory is short, and union mounts would typically be used
    > on such systems.

    (Sorry for the delay - I've been on vacation.)

    Hm, my intuition is that a tmpfs mount would be fairly lightweight in
    terms of memory - the main overhead over the barebones solution would
    be one superblock and vfsmount struct per mount. What am I missing?

    > The big reason why kernel impementation of readdir is hard is that
    > unswappable kernel memory needs to be used for caching directory
    > contents while the directory is open. Well, tmpfs does the same,
    > dentries and inodes are _not_ swappable, and they gobble up memory.

    That's a good point. It seemed to me that it wouldn't be too
    difficult to make those entries evictable - drop a reference count and
    set the ->d_release to mark the directory as needing rebuilding. What
    do you think?

    > So where's the advantage over implementing a thin deduplicating and
    > caching layer for union mounts?
    > Thanks,
    > Miklos


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