----- Original Message -----
From: "Eric Blake" <eblake@redhat.com> To: "Gergely Horváth" <gergely.horvath@inepex.com>, libvirt-users@redhat.com Sent: Wednesday, February 5, 2014 4:47:47 PM Subject: Re: [libvirt-users] Can I move the disk image of the guest while it is running?
On 02/05/2014 02:54 PM, Gergely Horváth wrote:
Thank you Eric,
On 2014-02-05 17:23, Eric Blake wrote:
Yes, live storage migration is possible; although at the moment, qemu is lacking a way to restart the operation if it fails midstream, so libvirt only allows the operation if you are willing to temporarily make your guest transient.
What does this mean? Will I loose anything if - for example - there is not enough space on the target device? Or it will still use the original disk image?
If blockcopy fails before mirroring is in sync, such as for insufficient space on the destination, then the source is still valid, with no data loss. If it reaches sync, then both source and destination are copies of each other until you quit the operation (the --wait flag tells virsh to automatically wait for sync, and the --pivot flag says to quit the operation as soon as sync is reached, rather than doing an indefinite mirror). As long as the guest doesn't quit in the meantime, you haven't lost anything. If the guest powers itself off in the middle of the blockcopy, the blockcopy will fail, but the original is still intact at the state it was when the guest shut down.
AFAIK, a transient guest only means it will disappear after the virtualisation session ends.
Correct, which is why you can then re-define the guest with the XML you saved earlier, so that it is once again persistent and can be safely powered off. If blockcopy --pivot succeeded, then the guest uses only the destination, and you have successfully gotten rid of the need to refer to the source.
virsh dumpxml $dom > file virsh undefine $dom virsh blockcopy $dom /ssd/image.raw /hdd/image.raw \ --wait --verbose --pivot virsh define file
Oh, I just realized a caveat - before redefining the file, you'll need to edit it to reflect a successful blockcopy (otherwise, if the guest stops and then is rebooted, the reboot will still use the original source, rather than the destination); a safer set of steps might be:
virsh dumpxml $dom > file virsh undefine $dom if virsh blockcopy $dom /ssd/image.raw /hdd/image.raw \ --wait --verbose --pivot; then virsh dumpxml $dom > file # refresh the xml to reflect new disk source fi virsh define file
so that file restores you to the original disk location if blockcopy fails, but tracks the new location if blockcopy succeeds.
[In my development work, I've been testing on throwaway disks, where I don't care if I clobber things by starting over again - but in a production environment, it pays to be more careful, and possibly to experiment on a safe domain with throwaway disk before doing it on your real domain]
I could not find anything about "pivot" or "pivoting"? What does --pivot do in this case?
--pivot says to break mirroring by using just the destination (in other words, pivot away from the source). The alternative is to use --finish, which says break mirroring by using just the source (in other words, the destination is now a point-in-time snapshot at the time you broke mirroring - useful for backup purposes).
And if you are terrified of the possible consequences of a transient domain, there is another possibility for moving your data to a new disk, except that it forces you to convert to qcow2:
virsh snapshot-create-as $dom --no-metadata --disk-only \ --diskspec /ssd/image.raw,file=/hdd/image.qcow2 virsh blockpull $dom --wait --verbose
and also has the drawback that if the guest quits in the middle of the blockpull, then you have to track both the original file and the snapshot wrapper - but at least blockpull is restartable from where it left off, for the next time the guest is running, whereas blockcopy has to start from scratch.
Eric, For scripted live backups, which non-transient technique would you prefer: 1. snapshot-create-as followed by blockpull, which results in the original file being left as a standalone copy of the VM from that point in time; it can then be copied off to a backup disk once blockpull completes 2. snapshot-create-as followed by copying the backing file (since it is now read-only) and then using blockcommit to pull the interim changes back into the original/backing file (and removing the snapshot file) For #2 I suppose you could hash the backing file and the copy once it finishes to verify integrity of the backup copy. Thanks, Andrew