[libvirt-users] qemu:///session and network-mounted home directories

[Spencer Baugh]

Cole Robinson <crobinso at redhat.com> writes:
> The proper way to make sure shared VMs aren't started across multiple machines
> is libvirt's locking support: https://libvirt.org/locking.html
>
> It requires running a separate daemon though so isn't trivial, and I have no
> idea if it can be made to work with qemu:///session.

Hmm, but what do you mean by shared VMs here? Just shared disk images,
or also shared configuration, or also marked autostart on multiple
hosts?

It seems like there are two closely related problems here:
1. For some shared VM and host, should we start that VM on that host?
2. Preventing VMs from running on multiple hosts at the same time

Problem 2 is definitely solved pretty well by libvirt's locking support;
even if that locking doesn't currently work properly with
qemu:///session, I accept the idea that the correct answer is to get
that working.

And problem 1 is solved as a side effect of solving problem 2, because
now one can just mark the VMs autostart on all hosts and let them race
to get a lock on the disk image, arbitrarily distributing them over the
hosts.

But surely there is a better way to solve problem 1 than that!

I suppose one could punt that question to a cross-host VM scheduler like
OpenStack. In that case I guess what one might do is not mark anything
autostart, and have the external scheduler come in on bootup and kick
specific things into action. Then you would rely on the cross-host
scheduler knowing what should run where.

But that doesn't really scale elegantly between the one-host and
multi-host cases, since in the one-host case it would be somewhat
awkward and unnecessary to use a multi-host scheduler.

Hmm, I suppose if the external scheduler instead specified a mapping
from /etc/machine-id to a list of autostarted VMs, that mapping could be
shared on all hosts, and so would work for both cases rather nicely.

Alternatively, even better: the autostart mechanism could be made
generic, allowing multiple ways to get the list of VMs that should be
autostarted. Then the most simple way (certainly not the best way) to
get the desired functionality would be to write a script that returns a
list of VMs that should be running on the current host.

Does that seem like a plausible approach?

(This would definitely work well with my future plan to write a simple,
minimal multi-host scheduler that can work with qemu:///session...)

> It's an interesting problem though, but not one that I can see libvirt adding
> explicit autostart+machine-id support to handle since it's pretty
> niche.

Hmm, well, I was going to say that that was just a throwaway idea, but
I've actually convinced myself that it might be a good idea now. Since
it has some pretty good applicability to actual qemu:///system stuff.

> Maybe you can disable autostart at the libvirt level, and add a custom
> ~/bin/virsh wrapper to do the manually autostart VMs only for the
> machine-id you care about, or something similarly crazy

Yeah, I could definitely do some local hack to get things working
nicely, but local hacks are what I am trying to avoid...