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Re: [libvirt] [PATCH] qemu: never send SIGKILL to qemu process unless specifically requested



On 01/30/2012 06:02 AM, Daniel P. Berrange wrote:
On Fri, Jan 27, 2012 at 01:35:35PM -0500, Laine Stump wrote:
When libvirt is shutting down the qemu process, it first sends
SIGTERM, then waits for 1.6 seconds and, if it sees the process still
there, sends a SIGKILL.

There have been reports that this behavior can lead to data loss
because the guest running in qemu doesn't have time to flush it's disk
cache buffers before it's unceremoniously whacked.

One suggestion on how to solve that problem was to remove SIGKILL from
the normal virDomainDestroyFlags, but still provide the ability to
kill qemu with SIGKILL by using a new flag to virDomainDestroyFlags.
This patch is a quick attempt at that in order to start a
conversation on the topic.

So what are your opinions? Is this the right way to solve the problem?
No, we can't change the default semantics of virDomainDestroy in
this case. Applications expect that we do absolutely everything
possible to kill of the guest. This is particularly important for
cluster fencing usage. If we only use SIGTERM, then we're introducing
unacceptable risk to apps relying on this.

We could do the opposite though - have a flag to do a gracefully
destroy, leaving the default as un-graceful.

virDomainShutdown ends up calling qemuProcessKill() too. So, I guess we need to add a flag there too.

In the meantime, shouldn't we at least wait longer before resorting to SIGKILL? (especially since it appears the current timeout is quite often too short). (If we don't at least do that, what we're saying is "the behavior of virDomainShutdown / virDomainDestroy is to lose your data unless you're lucky. If you don't want this behavior, you need to use virDomainXXXFlags, and specify the VIR_DOMAIN_DONT_TRASH_MY_DATA flag" :-P).


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