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

[Avi Kivity]

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.

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