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

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.