On 12/28/2016 12:51 PM, Christopher Pereira wrote: > Hi Eric, > > There is something I don't understand. > > We are doing: "virsh save", "qemu-img convert", "qemu-img rebase" and > "virsh restore". > We only touch the backing chain by doing the rebase while the VM is down. > Is there any chance this procedure can destroy data?
Since there are never two active writers to the file (you have made sure that qemu-img and qemu are not both holding the same file open at once), there is no chance for corruption on that front. There's still a possibility of corruption if you botch the instructions, and don't rebuild the chain in such a way that the guest would see the same data, but that's harder to say without seeing an actual transcript. > If so, is there any difference between shutting down and just > saving/restoring the VM? Shutting down vs. saving/restoring shouldn't matter other than the length of time that the guest is offline (including rebooting time with a shut down, which is not present for an incoming migration restoration). > Maybe save/restore keeps a cache? No. Both shutting down and saving end the qemu process, and therefore flush everything to the disk. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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