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From: Stefan Hajnoczi <stefanha@redhat.com>
To: "Benoît Canet" <benoit.canet@irqsave.net>
Cc: pbonzini@redhat.com, dietmar@proxmox.com, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command
Date: Tue, 3 Sep 2013 09:54:31 +0200	[thread overview]
Message-ID: <20130903075431.GA1610@stefanha-thinkpad.redhat.com> (raw)
In-Reply-To: <20130902125722.GG5073@irqsave.net>

On Mon, Sep 02, 2013 at 02:57:23PM +0200, Benoît Canet wrote:
> 
> I don't see the point of using hashes.
> Using hashes means that at least one extra read will be done on the target to
> compute the candidate target hash.
> It's bad for a cloud provider where IOs count is a huge cost.
> 
> Another structure to replace a bitmap (smaller on the canonical case) would be
> a block table as described in the Hystor paper:
> www.cse.ohio-state.edu/~fchen/paper/papers/ics11.pdf

This is similar to syncing image formats that use a revision number for
each cluster instead of a hash.

The problem with counters is overflow.  In the case of Hystor it is not
necessary to preserve exact counts.  A dirty bitmap must mark a block
dirty if it has been modified, otherwise there is a risk of data loss.

A bit more than just counters are necessary to implement a persistent
dirty bitmap, but maybe it's possible with some additional state.

Stefan

  reply	other threads:[~2013-09-03  7:54 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-09-02 12:57 [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command Benoît Canet
2013-09-03  7:54 ` Stefan Hajnoczi [this message]
  -- strict thread matches above, loose matches on Subject: below --
2013-05-15 14:34 Stefan Hajnoczi
2013-05-16  6:16 ` Wenchao Xia
2013-05-16  7:47   ` Stefan Hajnoczi
2013-05-17  6:58     ` Wenchao Xia
2013-05-17  9:14       ` Stefan Hajnoczi
2013-05-21  3:25         ` Wenchao Xia
2013-05-21  7:34           ` Stefan Hajnoczi
2013-05-17 10:17     ` Paolo Bonzini
2013-05-20  6:24       ` Stefan Hajnoczi
2013-05-20  7:23         ` Paolo Bonzini
2013-05-21  7:31           ` Stefan Hajnoczi
2013-05-21  8:30             ` Paolo Bonzini
2013-05-21 10:34               ` Stefan Hajnoczi
2013-05-21 10:36                 ` Paolo Bonzini
2013-05-21 10:58                   ` Dietmar Maurer
2013-05-22 13:43                     ` Stefan Hajnoczi
2013-05-22 15:10                       ` Dietmar Maurer
2013-05-22 15:34                       ` Dietmar Maurer
2013-05-23  8:04                         ` Stefan Hajnoczi
2013-05-23  8:11                           ` Dietmar Maurer
2013-05-24  8:38                             ` Stefan Hajnoczi
2013-05-24  9:53                               ` Dietmar Maurer

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