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[libvirt] libvirt(-java): virDomainMigrateSetMaxDowntime



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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