[libvirt] RFC backup API

[John Snow]

On 03/23/2016 06:36 AM, Kashyap Chamarthy wrote:
> On Mon, Mar 21, 2016 at 01:18:19PM +0300, Maxim Nestratov wrote:
>> 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.
> 
> There's also the 'blockdev-backup' command, which seems similar in
> operation to 'drive-backup', but differs subtly.
> 
> Looking at qmp-commands.hx, I learn that 'blockdev-backup' accepts
> target ID; while 'drive-backup' accept target drive name, otherwise,
> their operation look almost identical, and both commands use
> backup_start() (from qemu/blockdev.c).  [Added John Snow in CC to
> correct me if I'm wrong.]
> 

No, you're right. Blockdev-backup can backup to an arbitrary device
(which can be backed by a new file), but drive-backup will only accept a
new file.

I don't think blockdev-backup supports incremental backups just yet, but
I don't think there's any reason it can't. (Looking at it: yeah, why
have I not done that yet?...)

> For 'blockdev-backup'
> ---------------------
> 
> -> { "execute": "blockdev-backup", "arguments": { "device": "src-id",
>                                                   "sync": "full",
>                                                   "target": "tgt-id" } }
> <- { "return": {} }
> 
> 
> Where 'tagert' in this case means:
> 
>     "the name of the backup target device. (json-string)"
> 
> 
> For 'drive-backup'
> -----------------
> 
> -> { "execute": "drive-backup", "arguments": { "device": "drive0",
>                                                "sync": "full",
>                                                "target": "backup.img" } }
> <- { "return": {} }
> 
> Here, 'target' means:
> 
>         "the target of the new image. If the file exists, or if it is a
>         device, the existing file/device will be used as the new
>         destination.  If it does not exist, a new file will be created.
>         (json-string)"
> 
> [...]
> 
> 

--js