Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Chris Friesen]

On 07/18/2014 09:54 AM, Andrey Korolyov wrote:
> On Fri, Jul 18, 2014 at 6:58 PM, Chris Friesen
> <address@hidden> wrote:
> > Hi,
> >
> > I've recently run up against an interesting issue where I had a number of
> > guests running and when I started doing heavy disk I/O on a virtio disk
> > (backed via ceph rbd) the memory consumption spiked and triggered the
> > OOM-killer.
> >
> > I want to reserve some memory for I/O, but I don't know how much it can use
> > in the worst-case.
> >
> > Is there a limit on the number of in-flight I/O operations?  (Preferably as
> > a configurable option, but even hard-coded would be good to know as well.)
> >
> > Thanks,
> > Chris
>
> Hi, are you using per-vm cgroups or it was happened on bare system?
> Ceph backend have writeback cache setting, may be you hitting it but
> it must be set enormously large then.

This is without cgroups.  (I think we had tried cgroups and ran into 
some issues.)  Would cgroups even help with iSCSI/rbd/etc?

The "-drive" parameter in qemu was using "cache=none" for the VMs in 
question.  But I'm assuming it keeps the buffer around until acked by 
the far end in order to be able to handle retries.

Chris