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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