Open Stack

Hi Ignazio,

Is it the actual live-migration that takes long (e.g. the libvirt migration you can watch with “virsh domjobinfo <instance>”) or the whole live-migration process as observed by nova.

We have seen it a few times that the thing that actually takes long is plugging the neutron port on the target hypervisor (although I think this only applies to ml2-ovs). For us this seems to happen because the neutron-openvswitch-agent can take some time to assemble the firewall rules for the security group of the port (especially if you use large remote security groups).
This would also explain why migrating back is fast, because the neutron-openvswitch-agent on the source will have the information cached.

Alternatively you could have multiple live-migrations queued for the same source hypervisor, but nova only handles them one-by-one (unless you set max_concurrent_live_migrations).

--
Felix Huettner

From: Ignazio Cassano <ignaziocassano at gmail.com>
Sent: Wednesday, August 3, 2022 12:28 PM
To: openstack-discuss <openstack-discuss at lists.openstack.org>
Subject: [openstack] how to speed up live migration?

Hello All,
I am looking for a solution to speed up live migration.
Instances where ram is used heavily like java application servers, live migration take a long time (more than 20 minutes for 8GB ram instance) and converge mode is already set to True in nova.conf.
I also tried with post_copy but it does not change.
After the first live migration (very solow) if I try to migrate again it is very fast.
I presume the first migration is slow because memory fragmentation when an instance is running on the same compute node for a long time.
I am looking for a solution considering the on my computing node I can have
a little ram overcommit. Any case I am increasing the number of compute nodes to reduce it.
Thanks
Ignazio


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