On Mon, Apr 16, 2012 at 23:05:58 -0600, Eric Blake wrote:
For now, disk migration via block copy job is not implemented. But when we do implement it, we have to deal with the fact that qemu does not provide an easy way to re-start a qemu process with mirroring still intact (it _might_ be possible by using qemu -S then an initial 'drive-mirror' with disk reuse before starting the domain, but that gets hairy). Even something like 'virDomainSave' becomes hairy, if you realize the implications that 'virDomainRestore' would be stuck with recreating the same mirror layout.
But if we step back and look at the bigger picture, we realize that the initial client of live storage migration via disk mirroring is oVirt, which always uses transient domains, and that if a transient domain is destroyed while a mirror exists, oVirt can easily restart the storage migration by creating a new domain that visits just the source storage, with no loss in data.
We can make life a lot easier by being cowards, and forbidding certain operations on a domain. This patch guarantees that we never get in a state where we would have to restart a domain with a mirroring block copy, by preventing saves, snapshots, hot unplug of a disk in use, and conversion to a persistent domain (thankfully, it is still relatively easy to 'virsh undefine' a running domain to temporarily make it transient, run tests on 'virsh blockcopy', then 'virsh define' to restore the persistence).
The change to qemudDomainDefine looks a bit odd for undoing an assignment, rather than probing up front to avoid the assignment, but this is because of how virDomainAssignDef combines both a lookup and assignment into a single function call.
* src/conf/domain_conf.h (virDomainHasDiskMirror): New prototype. * src/conf/domain_conf.c (virDomainHasDiskMirror): New function. * src/libvirt_private.syms (domain_conf.h): Export it. * src/qemu/qemu_driver.c (qemuDomainSaveInternal) (qemuDomainSnapshotCreateXML, qemuDomainRevertToSnapshot) (qemuDomainBlockJobImpl, qemudDomainDefine): Prevent dangerous actions while block copy is already in action. * src/qemu/qemu_hotplug.c (qemuDomainDetachDiskDevice): Likewise. ---
was 8/18 in v4 v5: improve commit message
src/conf/domain_conf.c | 12 ++++++++++++ src/conf/domain_conf.h | 1 + src/libvirt_private.syms | 1 + src/qemu/qemu_driver.c | 29 +++++++++++++++++++++++++++++ src/qemu/qemu_hotplug.c | 7 +++++++ 5 files changed, 50 insertions(+), 0 deletions(-)
OK, nice commit message improvement. Jirka