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

[Paolo Bonzini]

Il 18/07/2014 18:22, Chris Friesen ha scritto:
> On 07/18/2014 09:24 AM, Paolo Bonzini wrote:
>> Il 18/07/2014 16:58, Chris Friesen ha scritto:
>>>
>>> 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.)
>>
>> For rbd, there is no such limit in QEMU except the size of the virtio
>> ring buffer, but librbd may add limits of its own.
>>
>> For files, there's no limit if you use aio=threads, but the more I/O
>> operations you trigger the more threads QEMU will create.  After 10
>> seconds of idle the threads will be destroyed.
>>
>> Also for files, the limit is 128 per disk if you use aio=native.  You
>> can only change it by recompilation only.
> 
> Has anyone looked at enforcing some limits?  I'm okay with throttling
> performance if necessary, but I really don't want to trigger the
> OOM-killer.

I forgot about "-drive ...,iops_max=NNN". :)

Paolo