Re: KVM performance vs. Xen

[Avi Kivity <avi@redhat.com>]

Andrew Theurer wrote:
>
>>
>>> disk:    read: 17 MB/sec  write: 40 MB/sec
>>
>> This could definitely cause the extra load, especially if it's many 
>> small requests (compared to a few large ones).
> I don't have the request sizes at my fingertips, but we have to use a 
> lot of disks to support this I/O, so I think it's safe to assume there 
> are a lot more requests than a simple large sequential read/write.

Yes.  Well the high context switch rate is the scheduler's way of 
telling us to use linux-aio.  If "lot's of disks" == 100, with a 3ms 
seek time, that's already 60,000 cs/sec.

>> Really, I think linux-aio support can help here.
> Yes, I think that would work for real block devices, but would that 
> help for files?  I am using real block devices right now, but it would 
> be nice to also see a benefit for files in a file-system.  Or maybe I 
> am mis-understanding this, and linux-aio can be used on files?

It could work with files with cache=none (though not qcow2 as now written).

-- 
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