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Subject: Re: [Qemu-devel] is there a limit on the number of in-flight I/O
 operations?
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To: Andrey Korolyov <andrey@xdel.ru>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On 07/18/2014 09:54 AM, Andrey Korolyov wrote:
> On Fri, Jul 18, 2014 at 6:58 PM, Chris Friesen
> <chris.friesen@windriver.com> 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