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Re: [Qemu-devel] [Qemu-block] [RFC] Differential Backups

From: John Snow
Subject: Re: [Qemu-devel] [Qemu-block] [RFC] Differential Backups
Date: Tue, 05 May 2015 11:55:49 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 05/05/2015 06:25 AM, Stefan Hajnoczi wrote:
On Wed, Apr 29, 2015 at 06:51:08PM -0400, John Snow wrote:
This is a feature that should be very easy to add on top of the existing
incremental feature, since it's just a difference in how the bitmap is

- Links to the last incremental (managed by libvirt)
- Clears the bitmap after creation

- Links to the last full backup always (managed by libvirt)
- Does not clear the bitmap after creation

No biggie.

Differential backups can be done using incremental backup functionality
in QEMU:

The client application points QEMU to the same target repeatedly instead
of keeping separate incremental backups.


Oh, so you're saying:


And then when making a new incremental, we re-use diff1 as a target and overwrite it so that it becomes:


In effect giving us a differential.

OK, so it's possible, but we still lose out on some flexibility that a slightly different mode would provide us, like the ability to keep multiple differentials if desired. (Well, I suppose we *can* create those by manually copying differentials after we create them, but that seems hackier than necessary.)

Still, it would be such a paltry few lines of code and introduce no real complexity to the subsystem, and it might make libvirt's time a little easier for managing such things.


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