[fedora-virt] I/O load distribution

Richard W.M. Jones rjones at redhat.com
Mon Jul 27 16:47:08 UTC 2009

On Mon, Jul 27, 2009 at 03:15:05PM +0200, Dennis J. wrote:
> What is the best way to deal with I/O load when running several VMs on a  
> physical machine with local or remote storage?
> What I'm primarily worried about is the case when several VMs cause disk  
> I/O at the same time. One example would be the "updatedb" cronjob of the  
> mlocate package. If you have say 5 VMs running on a physical System with 
> a local software raid-1 as storage and the all run updatedb at the same 
> time that causes all of them to run really slowly because the starve each 
> other fighting over the disk.
> What is the best way to soften the impact of such a situation? Does it 
> make sense to use a hardware raid instead? How would the raid type affect 
> the performance in this case? Would the fact that the I/O load gets 
> distributed across multiple spindles in, say, a 4 disk hardware raid-5 
> have a big impact on this?
> I'm currently facing the problem where I fear that random disk I/O by too 
> many VMs on a physical system could cripple their performance even though 
> I have plenty of CPU cores/RAM left to run them.
> Has anyone experience with this problem and maybe some data to shed some  
> light on this potential bottleneck for virtualization?

For KVM, each VM isn't really any different from a host process, so
you have to deal with them the same way that you'd deal with having
lots of host processes doing I/O.  So: lots of spindles, expensive I/O
hardware, etc.


Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
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