[libvirt] RFC backup API

[Maxim Nestratov]

Hi all,

It's been already quite a long time since qemu implemented QMP 
"drive-backup" command to create block devices backups. Even more, since 
qemu 2.4 there is a possibility to create incremental backups. Though it 
is possible to backup all attached to a domain disk drives by combining 
them into a single QMP transaction command, this way of creating them, 
not to mention managing, remains inconvenient for an end user of 
libvirt. Moreover, creating a single drive backup via QMP interface 
isn't handy either. That said, it looks reasonable to introduce a *new 
backup API* based on QMP "drive-backup" facilities.

Though we can start from a single simple function, allowing to create a 
disk backup by means of QMP "drive-backup" command, I'd like to discuss 
here the level of management libvirt could provide for backup 
operations. To begin with, here is the preliminary list of possible 
functions that I think make sense for libvirt API.

virDomainCreateBackup - which creates a backup full/incremental of 
all/selected disks,
virListBackups - which lists all backups created for a particular 
domain/target,
virRestoreBackup - which restores all/selected disks from a backup,
virDeleteBackup - which deletes all/selected disks from a backup.

It looks like backup management functions,  except create one, shouldn't 
be or might not be bound to a particular domain and we could possibly 
leverage storage pool API with some extension. Specifically, volume 
definition could be extended with necessary meta data related to backups.

The *question* is: if the whole idea about this new API as described 
above or something similar makes sense?

If yes, then let's find out requirements for it (if any) and I will try 
to prepare a patch set with the first RFC implementation to discuss the 
API in more details. Looking forward for your opinions on the matter.

Maxim