Re: how to tweak kernel to get the best out of kvm?

[Avi Kivity <avi@redhat.com>]

On 03/11/2010 03:24 PM, Harald Dunkel wrote:
> Hi Avi,
>
> I had missed to include some important syslog lines from the
> host system. See attachment.
>
> On 03/10/10 14:15, Avi Kivity wrote:
>    
>> You have tons of iowait time, indicating an I/O bottleneck.
>>
>>      
> Is this disk IO or network IO?

disk.

> The rsync session puts a
> high load on both, but actually I do not see how a high
> load on disk or block IO could make the virtual hosts
> unresponsive, as shown by the hosts syslog?
>
>    

qcow2 is still not fully asynchronous, so sometimes when it waits, a 
vcpu waits as well.

>> Here the problem is likely the host filesystem and/or I/O scheduler.
>>
>> The optimal layout is placing guest disks in LVM volumes, and accessing
>> them with -drive file=...,cache=none.  However, file-based access should
>> also work.
>>
>>      
> I will try LVM tomorrow, when the test with reiserfs is completed.
>
>    

If the slowdown is indeed due to I/O, LVM (with cache=off) should 
eliminate it completely.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.