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Re: [libvirt] [PATCH] qemu: Allow graceful domain destroy

On 08/22/2011 07:22 AM, Eric Blake wrote:
On 08/22/2011 04:49 AM, Michal Privoznik wrote:

I don't think that's problem. High priority calls must be guaranteed to
end in reasonably short time. And although we talk to monitor here, we
are guaranteed to end. Therefore no need to change my previous patch.

No, we are not guaranteed to end in a reasonably short time. Monitor
calls are, by nature, indefinite time commands, because they require
response from an external process.

Rather, we should add virDomainShutdownFlags() (which would be low
priority, to match virDomainShutdown() being low priority), and make
this a flag an option to virDomainShutdownFlags(). That is, you get a
choice between shutdown via ACPI signal, or shutdown via monitor
command, but both choices involve interaction with the monitor, and are
therefore inherently liable to blocking (although shutdown via the
monitor is much more likely to succeed in a short amount of time if the
monitor is available, because it does not depend on the guest's reaction
to ACPI).

'Accessing monitor' is a bit unfortunate formulation, because 99% calls
which do access monitor can block indefinitely. And this is the real
problem. Stuck API. It doesn't really matter if it is because of
monitor, NFS, etc. I just wanted to provide guide for developers if they
are in doubt whether to mark API as high or low priority.

The point is that virDomainDestroy() can't make a monitor command, or it
will get stuck. I don't think it makes sense to give
virDomainDestroyFlags() a flag that it can only honor in cases where the
monitor is not stuck, but which gets skipped otherwise. I think that if
we support a flag, then the use of the monitor has to be unconditional
with that flag, which in turn implies that we need

I wrote the above before even glancing at your patch. But now, looking at the patch, it looks like you tried to take this into consideration - you are trying to add condition variables to guarantee that the monitor command will be attempted if possible, but that the command will give up on an unresponsive monitor and resort to the more drastic process kill, so that the command will always succeed even in the face of a stuck monitor.

Maybe your approach has merit after all. But in that case, I think we need two ways to expose the monitor command:

virDomainDestroyFlags(, VIR_DOMAIN_DESTROY_FLUSH_CACHE) - best effort try to flush cache, but may still lose data because the destroy is guaranteed to happen after x seconds even if monitor could not be obtained

virDomainShutdownFlags(, VIR_DOMAIN_SHUTDOWN_FLUSH_CACHE) - guarantee that the flush cache happens, or that the command fails due to a timeout in obtaining the monitor

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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