libvirt users - Oct 2015 - Re: qemu:///session and network-mounted home directories

Dear libvirt-users,

Suppose my home directory is network mounted on to several different
machines. And suppose I defined some VMs in qemu:///session. The
configuration files and disk images for qemu:///session are stored in my
home directory.

Now suppose I log in to some machine and use virsh. The qemu:///session
VMs that are marked autostart are started. Now if I log into another
machine and use virsh, it will presumably try to start my autostart VMs
again, since the same configuration is present in both homedirs.

Is there anything libvirt does that might prevent it from starting the
same VM multiple times on different machines? Or might allow
machine-specific autostart configuration? Maybe something could be done
with /etc/machine-id!

In terms of qemu:///system, this is like having /var/lib/libvirt/images/
live on a network-mounted filesystem (fairly normal I assume) and also
having /etc/libvirt/qemu live on a network-mounted filesystem (not as
normal). Not sure how you would deal with the issue there either but I
can see why you might want to have your same VM configuration available
on all hosts, so it would be cool to figure out that problem as
well.

The answer may be the VM configuration is host-specific, so sharing VM
configuration between hosts is not a good idea. In that case, any other
recommendations on how to deal with the networked homedir case?

Thanks!

On 10/10/2015 10:53 AM, Spencer Baugh wrote:
> 
> Dear libvirt-users,
> 
> Suppose my home directory is network mounted on to several different
> machines. And suppose I defined some VMs in qemu:///session. The
> configuration files and disk images for qemu:///session are stored in my
> home directory.
> 
> Now suppose I log in to some machine and use virsh. The qemu:///session
> VMs that are marked autostart are started. Now if I log into another
> machine and use virsh, it will presumably try to start my autostart VMs
> again, since the same configuration is present in both homedirs.
> 
> Is there anything libvirt does that might prevent it from starting the
> same VM multiple times on different machines? Or might allow
> machine-specific autostart configuration? Maybe something could be done
> with /etc/machine-id!
> 
> In terms of qemu:///system, this is like having /var/lib/libvirt/images/
> live on a network-mounted filesystem (fairly normal I assume) and also
> having /etc/libvirt/qemu live on a network-mounted filesystem (not as
> normal). Not sure how you would deal with the issue there either but I
> can see why you might want to have your same VM configuration available
> on all hosts, so it would be cool to figure out that problem as
> well.
> 
> The answer may be the VM configuration is host-specific, so sharing VM
> configuration between hosts is not a good idea. In that case, any other
> recommendations on how to deal with the networked homedir case?
> 
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.

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.
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

- Cole

Cole Robinson <crobinso@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...