[libvirt] Limit autostarting of domains to a few at a time.

[Peter Krempa]

On 11/05/12 13:32, Ruben Kerkhof wrote:
> Hi list,

Hi Ruben,
>
> I have about a hundred kvm vm's on a physical host, all marked as autostart.
> As soon as libvirtd starts up, all vms get started and they start
> fighting for disk I/O.

> Would it be possible to add an option to wait a while before starting
> the next vm?
> Batch-starting them in parallel, let's say 10 at a time, would be great too.

The problem with this is how to detect if the machine is actually 
started up. For libvirt the guests are black boxes and we don't have 
insight if the guest has already started up. There are a few options how 
we could detect this, but none of them are ideal:
1) wait until I/O load calms down - this is impossible with I/O heavy guests

2) use guest agent - no way to know if it has actually started/is installed

3) wait a certain amount of time - some guests take longer to start

>
> I know I could use the libvirt-guest init script to do this, but we
> rely on the autostart property for several purposes.

The libvirt-guests script isn't 100% ideal for this purpose. It has the 
ability to re-start guests that were shut down by it, but can't be 
configured to start arbitrary guests.

Unfortunately autostarting of guest can't be disabled in the daemon. If 
it was possible you could "virsh list --uuid --inactive --autostart" 
guests that need to be autostarted and use the function from 
libvirt-guests that is used to start the machines to do so (instead of 
reading them from the save-file).

This would still use the non-ideal approach (3) to rate-limit the 
starting. With the knowledge of the guest, this could be improved.

As of the ability to disable autostarting, I'll have a look if it's hard 
to implement. I think this would make sense to have it as a config 
option. On the other hand, there's no ideal way to implement the 
detection of the end-of-boot event on the guest, so it'll probably be 
better to leave piece of functionality to be implemented by the users 
who know their infrastructure.

Until then, you'll have either to start them via libvirt-autostart 
without the ability to rate-limit starting or note the autostart state 
into some other element (eg. the <metadata> element) and have a custom 
script that starts the guests.

>
> Kind regards,
>
> Ruben Kerkhof
>

Peter