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Re: [libvirt] RFC backup API

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.]

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.



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