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

[John Snow <jsnow@redhat.com>]

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
>> treated:
>>
>> Incremental
>> - Links to the last incremental (managed by libvirt)
>> - Clears the bitmap after creation
>>
>> Differential:
>> - 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.
>
> Stefan
>

Oh, so you're saying:

[anchor]<--[diff1]

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

[anchor]<--[diff2]

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.

--js