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Re: [libvirt] RFC backup API



23.03.2016 17:27, John Snow пишет:

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?...)

John,
Any chances to get this implemented? Just not to use two different commands in libvirt and start using 'blockdev-backup' right away for both full and incremental backups?

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


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