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

[Mikulas Patocka <mpatocka redhat com>]

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?

Mikulas