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Re: [dm-devel] Re: Was: [patch 1/1] md/dm: Reduce stack usage with stacked block devices



On Thu, 2006-01-12 at 14:32 -0500, Mike Snitzer wrote:
> On 1/11/06, Ming Zhang <mingz ele uri edu> wrote:
> > On Wed, 2006-01-11 at 20:54 +0000, Alasdair G Kergon wrote:
> 
> > > Efficient, unlikely with the current implemention - hence the need
> > > for a rewrite.
> >
> > >
> > > If you have 5 snapshots of the same origin, the current implemention
> > > takes 5 copies of the data when you change it. That approach is
> > > never going to scale well!  And it uses lots of memory.
> >
> > yes, you are right. is there any new rewrite action proposed or planned?
> 
> My understanding is that much of RedHat's Cluster Snapshot Block
> Device (CSNAP) core is a candidate for the dm-snapshot rewrite:
> http://sources.redhat.com/cluster/csnap/csnap.ps
> 
> Along with other improvements, performance would be addressed with a
> shared exception table (tree) for all snapshots (only one copy of each
> exception).  When I spoke with Daniel Phillips (author of CSNAP
> stuffs) a few months ago he said CSNAP is on the back burner until
> others realize just how bad the existing dm-snapshot is.  He is
> looking/waiting for others who are willing to contribute in moving
> CSNAP's snapshot improvements into dm-snapshot++.  Unfortunately I'd
> only be able to contribute testing at this time...

Thanks a lot for this information! Just have a quick glimpse on
abstract. Looks interesting.


> 
> Mike


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