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Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command |
Date: |
Thu, 23 May 2013 10:04:17 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, May 22, 2013 at 03:34:18PM +0000, Dietmar Maurer wrote:
> > That sounds like more work than a persistent dirty bitmap. The advantage
> > is that
> > while dirty bitmaps are consumed by a single user, the Merkle tree can be
> > used
> > to sync up any number of replicas.
>
> I also consider it safer, because you make sure the data exists (using hash
> keys like SHA1).
>
> I am unsure how you can check if a dirty bitmap contains errors, or is out of
> date?
>
> Also, you can compare arbitrary Merkle trees, whereas a dirty bitmap is
> always related to a single image.
> (consider the user remove the latest backup from the backup target).
One disadvantage of Merkle trees is that the client becomes stateful -
the client needs to store its own Merkle tree and this requires fancier
client-side code.
It is also more expensive to update hashes than a dirty bitmap. Not
because you need to hash data but because a small write (e.g. 1 sector)
requires that you read the surrounding sectors to recompute a hash for
the cluster. Therefore you can expect worse guest I/O performance than
with a dirty bitmap.
I still think it's a cool idea. Making it work well will require a lot
more effort than a dirty bitmap.
Stefan
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, (continued)
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/20
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Paolo Bonzini, 2013/05/20
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Paolo Bonzini, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Paolo Bonzini, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/21
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/22
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/22
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/22
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command,
Stefan Hajnoczi <=
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/23
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Stefan Hajnoczi, 2013/05/24
- Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command, Dietmar Maurer, 2013/05/24