[libvirt] [PATCH v4 0/8] bulk snapshot list/redefine (incremental backup saga)

[Eric Blake]

On 3/12/19 6:45 PM, Nir Soffer wrote:

> 
> When you plug a disk, you cannot use any of the bitmaps on the disk because
> you don't have any guarantee that the user did not attach the disk to
> another machine
> that modified the data in some way without updating the bitmaps. So we are
> going to
> delete a disk from checkpoints once you unplug it, and start from scratch
> when you
> plug a disk.

Correct - from libvirt's point of view, when a disk is hotplugged, the
next backup needs to perform a full backup rather than an incremental
backup on that disk, because that disk was not present in the machine on
the last checkpoint and therefore does not have bitmap coverage from the
last checkpoint to the present moment. But you don't necessarily have to
delete bitmaps from the qcow2 file to get this effect.  The fact that
libvirt stored a full <domain> at the time of the checkpoint is enough
for libvirt to tell that you are requesting a backup of a disk that did
not exist at the starting checkpoint, and therefore that disk needs a
full rather than incremental backup.


> 
> I think what we keep is enough to tell libvirt which bitmaps are needed for
> a backup.

The current design for virDomainBackup() is that you state which
checkpoint to start from, and then libvirt computes all bitmaps from
that checkpoint to the present in order to perform the incremental
backup, or complains that there is no sequence of bitmaps that covers
that length in time so you have to do a full backup instead.  You do not
tell libvirt 'perform a backup of the clusters represented in bitmaps B,
C, and D', but 'perform a backup of the clusters modified since time
T2'.  Libvirt then crawls the chain of checkpoint objects to find T2 and
verify which bitmaps between T2 and the present (if that be bitmaps B,
C, and D) that it needs to use to perform that differential backup.

> 
> We probably need to keep the vm configuration per checkpoint or at least
> ensure
> consistency, so once you started a backup the configuration cannot change
> until
> the backup ends.

And that configuration per checkpoint IS the <domain> subelement.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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