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Re: [libvirt] [PATCH 2/2] qemu: Always use iohelper for dumping domain core



On Tue, Feb 07, 2012 at 14:35:44 -0700, Eric Blake wrote:
> There are two situations where we save to file - one during core dump
> (which, as you point out, can be live where the guest continues to run
> after the dump, and therefore must be interruptible), and one during
> 'virsh save' or 'virsh managedsave' (which is currently non-live - we
> pause the guest, do the migration, then kill qemu; resuming requires
> starting a new qemu).
> 
> In both situations, my understanding is that qemu does throttling based
> on whether it gets EAGAIN from the fd it is writing to, and that it
> doesn't regain responsiveness if it doesn't get EAGAIN.  For the dump
> case, where we might have requested the live flag, we need the
> responsiveness.  I'm just worried whether the save case also needs to
> present an EAGAIN interface to qemu (it would be nice if we didn't have
> to rely on that, so that we could change the throttling speeds - after
> all, we had to revert the patch that provided unlimited throttling when
> saving to disk because qemu didn't handle it well).

I guess you're right. I actually didn't study qemu code to understand what
exactly it is doing but I think we could just pass the right non-blocking fd
in both cases to feel safer :-)

Jirka


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