| title: | Re PATCH 00 32 VFS based Union Mount V3 |
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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 its 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 - Ive 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.
Thats a good point. It seemed to me that it wouldnt 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 wheres the advantage over implementing a thin deduplicating and
caching layer for union mounts?
Thanks,
Miklos
-VAL
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