[dm-devel] Re: [PATCH 2 of 2] DM Snapshot: dont insert before existing chunk
Mikulas Patocka
mpatocka at redhat.com
Thu Sep 24 14:15:36 UTC 2009
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
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