Re: Re: trimmable dm-snapshot?

[Douglas McClendon <dmc.fedora@filteredperception.org>]

Mike Snitzer wrote:
> 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.

I wasn't asking about trimmable dm-snapshot-origin devices, only 
trimmable[1] dm-snapshot devices.

Thinking about snapshot-origin devices, what you say is a valid reason 
why such optimization is not remotely easy (or feasible at all).

But for snapshot devices, I think the optimization should be as simple 
and effective as I described it.  (until I discover some other aspect of 
dm that changes my understanding of things).

[1] http://lwn.net/Articles/293658/

-dmc