[libvirt-users] Blockpull behavior when interrupted

[Eric Blake]

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files to no longer represent a state that was ever seen by the guest,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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