Re: [Qemu-devel] [RFC 4/8] block: add block_backup QMP command

[Eric Blake]

On 03/09/2013 03:22 PM, Stefan Hajnoczi wrote:
> @block-backup
> 
> Start a point-in-time copy of a block device to a new destination.
> 

I'm trying to figure out how this is different from drive-mirror.  If I
understand correctly:

After starting drive-mirror, a write to the block device is also written
to the mirror, so that the destination sees the new data

After starting block-backup, a write to the block device flushes the old
data to the destination, so that the destination sees the old data

Timing-wise, I can accomplish a backup through either command, with the
following differences:

With drive-mirror, I start a job, wait for it to hit sync'd state, then
cancel the job. The copy is tied to the point where I cancel, and the
moment I cancel, I no longer have to worry about keeping the destination
writable (that is, the bulk of the copying is done prior to the point in
time).

With block-backup, I start a job, then wait for it to complete.  The
copy is tied to the point where I started the job, but as that may take
some time, I have to keep the destination writable until the job
completes (that is, the bulk of the work is done after the point in time).

The concept is indeed useful; more so if we can wire this into
'transaction' to capture multiple disks at the same point in time.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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