Basically, what I want to do is to run accurate backups without shutting
down and restarting the VM. Is this possible? If it isn't, I think the
only alternative is to make a script that shuts the vm down, copies it,
restarts the vm then runs its compression and backup-over-ssh routine.
It's not possible in the general case to take a snapshot of the
underlying image since as others have pointed out, it may not be
consistent on disk since there are still data/metadata from the guest's
filesystem that haven't made it out to disk.
Commercial hypervisors provide guest tools that allow a filesystem
quiesce/sync that lock-steps with external snapshotting machinery.
However, one option that could be worth investigating is using ZFS in
the guest with disk images backed by a zvol on the host. ZFS guarantees
on-disk consistency, and a zvol provides instantaneous snapshots. Worst
case is that some writes are not picked up at snapshot time, but that
seems preferable to having to force a full sync in the guest.
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