On 07/21/2016 03:05 PM, Andrew Martin wrote: > Hello, > > I use snapshot-create-as followed by blockpull when creating external > snapshots > of VMs. This works well, however I am curious about the behavior of blockpull > after an unexpected shutdown (or SIGKILL). If a blockpull is in progress and > an > unexpected power loss occurs, will the VM continue to reference the backing > file > for the parts of it that have not yet been copied? Or, will will the disk > image > no longer be usable?
Blockpull is non-destructive. If it is aborted early, you can reissue a blockpull command and it will safely resume the task of pulling data. The guest-visible data remains unchanged, regardless of how much or how little of the blockpull has completed. Of course, you cannot discard the backing file until the blockpull has completed. An aborted block commit, on the other hand, can cause individual backing files to no longer represent a state that was ever seen by the guest, but again can be restarted to be fully completed; and the guest-visible data from the active layer is never corrupted even if individual backing files within the chain are (temporarily) out of sync with reality. For more ideas on how to visualize things, check out my presentation at last year's KVM Forum: http://www.linux-kvm.org/images/5/5f/03x02-Eric_Blake-Backing_Chain_Management_in_QEMU_and_libvirt.pdf -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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