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Right now, it is appallingly easy to cause qemu disk snapshots to alter a domain then fail; for example, by requesting a two-disk snapshot where the second disk name resides on read-only storage. In this failure scenario, libvirt reports failure, but modifies the live domain XML in-place to record that the first disk snapshot was taken; and places a difficult burden on the management app to grab the XML and reparse it to see which disks, if any, were altered by the partial snapshot. This patch adds a new flag where implementations can request that the hypervisor make snapshots atomically; either no changes to XML occur, or all disks were altered as a group. If you request the flag, you either get outright failure up front, or you take advantage of hypervisor abilities to make an atomic snapshot. Of course, drivers should prefer the atomic means even without the flag explicitly requested. There's no way to make snapshots 100% bulletproof - even if the hypervisor does it perfectly atomic, we could run out of memory during the followup tasks of updating our in-memory XML, and report a failure. However, these sorts of catastrophic failures are rare and unlikely, and it is still nicer to know that either all snapshots happened or none of them, as that is an easier state to recover from. * include/libvirt/libvirt.h.in (VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC): New flag. * src/libvirt.c (virDomainSnapshotCreateXML): Document it. * tools/virsh.c (cmdSnapshotCreate, cmdSnapshotCreateAs): Expose it. * tools/virsh.pod (snapshot-create, snapshot-create-as): Document it.
LibVirt : simple API for virtualization Libvirt is a C toolkit to interact with the virtualization capabilities of recent versions of Linux (and other OSes). It is free software available under the GNU Lesser General Public License. Virtualization of the Linux Operating System means the ability to run multiple instances of Operating Systems concurrently on a single hardware system where the basic resources are driven by a Linux instance. The library aim at providing long term stable C API initially for the Xen paravirtualization but should be able to integrate other virtualization mechanisms if needed. Daniel Veillard <veillard@redhat.com>