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Date: Mon, 11 Mar 2013 15:27:34 +0100
From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 0/8] block: Live backup prototype
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To: Dietmar Maurer <dietmar@proxmox.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>

On Mon, Mar 11, 2013 at 09:26:26AM +0000, Dietmar Maurer wrote:
> > > > We track zero blocks at 4KB level to get small backup files. Restore
> > > > can handle those blocks differently, either pre-allocate or create
> > > > holes. That
> > > can even be a restore option.
> > >
> > > You are right, the behavior can be set at restore time.
> > >
> > > I'm curious how much of a win the 4 KB zero detection is on random
> > > Linux or Windows guest images.  Have you collected numbers?
> > 
> > Well, seems that it does not help much - it saves about 0.2% space. So maybe
> > we can remove that feature (and increase address space in the VMA format
> > instead).
> 
> Sorry, I need to correct myself - I had a bug in the code. 
> 
> I can see space reduction up to 4% using this feature. Considering the fact that it
> comes at no cost, it would be stupid to remove it.

Okay, looks like it's useful but not a huge win.

In the NBD approach pipelining writes (or discards) ought to make 4 KB
zero blocks usable without overhead.

Stefan