Re: [PATCH 00/32] VFS based Union Mount (V3)

[Valerie Aurora <vaurora@redhat.com>]

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

-VAL