[dm-devel] Re: [PATCH 2 of 2] DM Snapshot: dont insert before existing chunk

[Mike Snitzer <snitzer redhat com>]

On Thu, Sep 24 2009 at 10:15am -0400,
Mikulas Patocka <mpatocka redhat com> wrote:

> On Thu, 24 Sep 2009, Mikulas Patocka wrote:
> 
> > On Thu, 24 Sep 2009, Alasdair G Kergon wrote:
> > 
> > > On Wed, Sep 23, 2009 at 10:25:08AM -0500, Jon Brassow wrote:
> > > > Don't insert into hash before an existing chunk.
> > > > Most inserts are after an existing extent anyway so this code is used
> > > > very rarely.
> > >  
> > > Again, I'm not particularly keen on taking this one.  Some systems are very
> > > short of memory and everything we can save matters and we have no statistics
> > > about how "rare" this actually is.
> > > 
> > > Is it really that difficult to split into two when removing?
> > 
> > It is code bloat --- it would increase coding/reviewing/testing time (the 
> > splitting would have to be simulated because it happens rarely).
> > 
> > Also, it could lead to the situation that memory consumption starts 
> > growing when merging.
> > 
> > Mikulas
> 
> These backmerges happen if someone writes to the logical volume backwards. 
> Do you know any common usage scenario that does it?

I was able to reliably reproduce backmerges simply by taking a snapshot
of a 32G LV and then formatting that origin with ext3.  This resulted in
a ~9% copy-out to the snapshot.

AFAIK mkfs.ext3 does write backwards for updating block groups.  In
practice it clearly does because without disabling the insertion before
other chunks I would reliably BUG in clear_exception (as I shared
earlier today on dm-devel).

Mike