From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: trimmable dm-snapshot?
Date: Thu, 22 Oct 2009 10:20:50 -0400
Message-ID: <20091022142050.GB19699@redhat.com>
References: <4ADFCC0D.6000903@filteredperception.org>
Reply-To: device-mapper development <dm-devel@redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <dm-devel-bounces@redhat.com>
Content-Disposition: inline
In-Reply-To: <4ADFCC0D.6000903@filteredperception.org>
List-Unsubscribe: <https://www.redhat.com/mailman/listinfo/dm-devel>,
	<mailto:dm-devel-request@redhat.com?subject=unsubscribe>
List-Archive: <https://www.redhat.com/archives/dm-devel>
List-Post: <mailto:dm-devel@redhat.com>
List-Help: <mailto:dm-devel-request@redhat.com?subject=help>
List-Subscribe: <https://www.redhat.com/mailman/listinfo/dm-devel>,
	<mailto:dm-devel-request@redhat.com?subject=subscribe>
Sender: dm-devel-bounces@redhat.com
Errors-To: dm-devel-bounces@redhat.com
To: device-mapper development <dm-devel@redhat.com>
List-Id: dm-devel.ids

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