Hi Stefan,
For true virtio-fs migration, we need to migrate the daemon’s (back
end’s) state somehow. I’m addressing you because you had a talk on this
topic at KVM Forum 2021. :)
As far as I understood your talk, the only standardized way to migrate a
vhost-user back end’s state is via dbus-vmstate. I believe that
interface is unsuitable for our use case, because we will need to
migrate more than 1 MB of state. Now, that 1 MB limit has supposedly
been chosen arbitrarily, but the introducing commit’s message says that
it’s based on the idea that the data must be supplied basically
immediately anyway (due to both dbus and qemu migration requirements),
and I don’t think we can meet that requirement.
Has there been progress on the topic of standardizing a vhost-user back
end state migration channel besides dbus-vmstate? I’ve looked around
but didn’t find anything. If there isn’t anything yet, is there still
interest in the topic?
Of course, we could use a channel that completely bypasses qemu, but I
think we’d like to avoid that if possible. First, this would require
adding functionality to virtiofsd to configure this channel. Second,
not storing the state in the central VM state means that migrating to
file doesn’t work (well, we could migrate to a dedicated state file,
but...). Third, setting up such a channel after virtiofsd has sandboxed
itself is hard. I guess we should set up the migration channel before
sandboxing, which constrains runtime configuration (basically this would
only allow us to set up a listening server, I believe). Well, and
finally, it isn’t a standard way, which won’t be great if we’re planning
to add a standard way anyway.
Thanks!
Hanna
- vhost-user (virtio-fs) migration: back end state Hanna Czenczek
-
