Re: kvm-83 write performance raw

[Anthony Liguori <anthony@codemonkey.ws>]

Malinka Rellikwodahs wrote:
> On Mon, Mar 2, 2009 at 16:22, Anthony Liguori <anthony@codemonkey.ws> wrote:
>   
>> Malinka Rellikwodahs wrote:
>>     
>>>>> What version of kvm is this?  Is it kvm-68?  You'll have better luck
>>>>> with
>>>>> something newer than that.
>>>>>
>>>>>           
>>> kvm-83 is the one with the problem, kvm-68 is working correctly.
>>>
>>>       
>> kvm-68 and qcow2 both use cache=writeback by default which is less safe than
>> cache=writethrough which is now the default.
>>
>> But performance shouldn't be as bad as your seeing.
>>     
>
> Running the kvm-84 install on the qcow image as kvm -m 1024 -drive
> file=qcow,boot=on,cache=writethrough, I get similar performance to the
> raw performance.
>
> So it looks like with cache set to writethrough there's a big
> performance hit on this setup, any ideas where to look for that?
>   

cache=writeback is "fake" performance.   In theory, it should be better 
than native because it's being loose with data consistency.  So the real 
question is why is performance so bad.

It's probably got something to do with IDE.  It could be that kvm-84's 
IDE is pre-Avi's AIO implementation such that you're splitting up IDE 
requests into something small.  They should be submitted in parallel but 
maybe that's not working out well with your disk setup.

Regards,

Anthony Liguori

>> Regards,
>>
>> Anthony Liguori
>>