From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Hi,
This set attempts to make a race condition between migration and
drive-mirror (and other block users) soluble by allowing the migration
to be paused after the source qemu releases the block devices but
before the serialisation of the device state.
The symptom of this failure, as reported by Wangjie, is a:
_co_do_pwritev: Assertion `!(bs->open_flags & 0x0800)' failed
and the source qemu dieing; so the problem is pretty nasty.
This has only been seen on 2.9 onwards, but the theory is that
prior to 2.9 it might have been happening anyway and we were
perhaps getting unreported corruptions (lost writes); so this
really needs fixing.
This flow came from discussions between Kevin and me, and we can't
see a way of fixing it without exposing a new state to the management
layer.
The flow is now:
(qemu) migrate_set_capability pause-before-switchover on
(qemu) migrate -d ...
(qemu) info migrate
...
Migration status: pre-switchover
...
<< issue commands to clean up any block jobs>>
(qemu) migrate_continue pre-switchover
(qemu) info migrate
...
Migration status: completed
This has been tested with Jiri's libvirt at:
https://gitlab.com/jirkade/libvirt.git migration-pause
with:
migrate --live --copy-storage-all --verbose
The precopy flow is:
active->pre-switchover->device->completed
The postcopy flow is:
active->pre-switchover->postcopy-active->completed
Although the behaviour with postcopy only gets interesting when
we add something like Max's active-sync.
Dave
--
v4
Comment fix in 'migrate-continue' example (thanks Jiri)
v3
A couple of FIXUPs that had escaped v2's merge
v2
Pause *before* block inactivation (thanks Peter)
Rename state and capability to Dan+KWolf's combined suggestion
Dr. David Alan Gilbert (7):
migration: Add 'pause-before-switchover' capability
migration: Add 'pre-switchover' and 'device' statuses
migration: Wait for semaphore before completing migration
migration: migrate-continue
migrate: HMP migate_continue
migration: allow cancel to unpause
migration: pause-before-switchover for postcopy
hmp-commands.hx | 12 +++++++
hmp.c | 13 ++++++++
hmp.h | 1 +
migration/migration.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++--
migration/migration.h | 4 +++
qapi/migration.json | 30 ++++++++++++++++--
6 files changed, 144 insertions(+), 4 deletions(-)
--
2.13.6