James Harper <james.harper@bendigoit.com.au> writes:
I need to move a block device (Xen VM backing store) between sites with a minimum of downtime... my choices so far are: [dd/drbd...] Both sites are linked by openvswitch so they are effectively the same lan. Bandwidth is 20Mbits/second so it won't be lightning fast but the VM's are typically only 50GB in size which equates to around 7 hours + overheads.
drbd is probably reasonable if you're intending to leave it around -- for a one-shot move, I probably wouldn't bother (unless I was already a drbd guru). What I probably WOULD try is to lvm snapshot a day before, and sync that. It will be incomplete and incoherent, but you don't care because on the day you rsync --only-write-batch against the snapshot and then upload only the diff and apply it. Since only the changed blocks from the last 24h have changed, that ought to reduce the downtime. It means you'll need to leave the snapshot around beforehand, of course. It it goes tits-up, fall back to bare dd plan. Or, of course, you can do it at the file level instead of the filesystem level, in which case you really *can* just use rsync (modulo usual stuff like bootloader and RDBMSs). From context I'm assuming the guest is windows or something equally horrible from that perspective. end brain dump - sorry about the incoherency