Hi,
I'm facing some troubles with virDomainMigrate &
virDomainMigrateSetMaxDowntime. The core problem is that KVM's default
value for the maximum allowed downtime is 30ms (max_downtime in
migration.c, it's nanoseconds there; 0.12.3) which is too low for my VMs
when they're busy (~50% CPU util and above). Migrations then take
literally forever, I had to abort them after 15 minutes or so. I'm using
GBit Ethernet, so plenty bandwidth should be available. Increasing the
allowed downtime to 50ms seems to help, but I have not tested situations
where the VM is completely utilized. Anyways, the default value is too
low for me, so I tried virDomainMigrateSetMaxDowntime resp. the Java
wrapper function.
Here I'm facing a problem I can overcome only with a quite crude hack:
org.libvirt.Domain.migrate(..) blocks until the migration is done, which
is of course reasonable. So I tried calling migrateSetMaxDowntime(..)
before migrating, causing an error:
"Requested operation is not valid: domain is not being migrated"
This tells me that calling migrateSetMaxDowntime is only allowed during
migrations. As I'm migrating VMs automatically and without any user
intervention I'd need to create some glue code that runs in an extra
thread, waiting "some time" hoping that the migration was kicked off in
the main thread yet and then calling migrateSetMaxDowntime. I'd like to
avoid such quirks in the long run, if possible.
So my question: Would it be possible to extend the migrate() method
resp. virDomainMigrate() function with an optional maxDowntime parameter
that is passed down as QEMU_JOB_SIGNAL_MIGRATE_DOWNTIME so that
qemuDomainWaitForMigrationComplete would set the value? Or are there
easier ways?
Thanks and regards,
-t
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