[dm-devel] Re: trimmable dm-snapshot?

[Mike Snitzer <snitzer redhat com>]

On Wed, Oct 21 2009 at 11:05pm -0400,
Douglas McClendon <dmc fedora filteredperception org> wrote:

> Has anyone looked into the idea of dm-snapshots responding
> appropriately to trims from filesystems?
> 
> I.e. the efficiency problem of a dm-snapshotted ext filesystem
> having files created and then deleted?  I.e. in such a scenario,
> resources in the cow device end up taken that could be freed if the
> dm layer could efficiently respond to trim notifications by
> discarding any useless exceptions?
> 
> I've been poking around pondering whether an offline quick hack
> might be possible with libext2fs and enough knowledge of the on-disk
> persistent snapshot format.  I.e. just walk the exception chunks in
> the cow device, use libext2fs (sufficient? easiest way?) to
> determine whether all the fsblocks/sectors the chunk contains are
> all currently unneeded, and if so reclaiming that space (possibly by
> relocating the last exception. I'm still a distance from truly
> grokking the on-disk format along with the rest of the dm-snapshot
> and exception-store code).
> 
> Does any of this make sense?  Been looked at?  Seem like a
> reasonable avenue to pursue?

The snapshot must faithfully maintain a copy of the origin's data
relative to a particular point in time.  You can't use changes to the
origin (trim or any other change) to delete the exceptions that a
snapshot is already maintaining.  That would invalidate the whole intent
of the snapshot.

Mike