Re: [libvirt] [PATCH v2 06/10] blockcommit: document semantics of committing active layer

Now that qemu 2.0 allows commit of the active layer, people are attempting to use virsh blockcommit and getting into a stuck state, because libvirt is unprepared to handle the two-phase commit required by qemu. Stepping back a bit, there are two valid semantics for a commit operation: 1. Maintain a 'golden' base, and a transient overlay. Make changes in the overlay, and if everything appears to work, commit those changes into the base, but still keep the overlay for the next round of changes; repeat the cycle as desired. 2. Create an external snapshot, then back up the stable state in the backing file. Once the backup is complete, commit the overlay back into the base, and delete the temporary snapshot. Since qemu doesn't know up front which of the two styles is preferred, a block commit of the active layer merely gets the job into a synchronized state, and sends an event; then the user must either cancel (case 1) or complete (case 2), where qemu then sends a second event that actually ends the job. However, until commit e6bcbcd, libvirt was blindly assuming the semantics that apply to a commit of an intermediate image, where there is only one sane conclusion (the job automatically ends with fewer elements in the chain); and getting stuck because it wasn't prepared for qemu to enter a second phase of the job. This patch adds a flag to the libvirt API that a user MUST supply in order to acknowledge that they will be using two-phase semantics. It might be possible to have a mode where if the flag is omitted, we automatically do the case 2 semantics on the user's behalf; but before that happens, I must do additional patches to track the fact that we are doing an active commit in the domain XML. Later patches will add support of the flag, and once 2-phase semantics are working, we can then decide whether to relax things to allow an omitted flag to cause an automatic pivot. * include/libvirt/libvirt.h.in (VIR_DOMAIN_BLOCK_COMMIT_ACTIVE) (VIR_DOMAIN_BLOCK_JOB_TYPE_ACTIVE_COMMIT): New enums. * src/libvirt.c (virDomainBlockCommit): Document two-phase job when committing active layer, through new flag. (virDomainBlockJobAbort): Document that pivot also occurs after active commit. * tools/virsh-domain.c (vshDomainBlockJob): Cover new job. * src/qemu/qemu_driver.c (qemuDomainBlockCommit): Explicitly reject active copy; later patches will add it in. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- include/libvirt/libvirt.h.in | 12 ++++++---- src/libvirt.c | 55 +++++++++++++++++++++++++++++--------------- src/qemu/qemu_driver.c | 21 ++++++++++++++--- tools/virsh-domain.c | 3 ++- 4 files changed, 65 insertions(+), 26 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index b0f80a5..b61b01b 100644 --- a/src/qemu/qemu_driver.c +++ b/src/qemu/qemu_driver.c

@@ -15538,9 +15541,21 @@ qemuDomainBlockCommit(virDomainPtr dom, * process; qemu 2.1 is further improving active commit. We need * to start supporting it in libvirt. */ if (topSource == disk->src) { - virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s", - _("committing the active layer not supported yet")); - goto endjob; + /* We assume that no one will backport qemu 2.0 active commit + * to an earlier qemu without also backporting the block job + * ready event; but this makes sure of that fact */ + if (!virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKJOB_ASYNC)) { + virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s", + _("active commit not supported with this QEMU binary")); + goto endjob; + } + /* XXX Should we auto-pivot when COMMIT_ACTIVE is not specified? */

+ if (!(flags & VIR_DOMAIN_BLOCK_COMMIT_ACTIVE)) { + virReportError(VIR_ERR_INVALID_ARG, + _("commit of '%s' active layer requires active flag"), + disk->dst); + goto endjob; + } } if (!topSource->backingStore) {