Re: Avoiding I/O bottlenecks between VM's

[Marcelo Tosatti <mtosatti@redhat.com>]

Hi Alberto,

On Fri, Sep 19, 2008 at 11:26:09AM -0600, Alberto Treviño wrote:
> I am using KVM 69 and kernel 2.6.25.17 to host several VM's in a large 
> server.  So far, everything has been great.  Except I'm adding a Windows 
> Server VM that will run a SQL Server database.  A few times I've noticed 
> that I/O becomes a bottleneck for the VM and Windows VM freezes for a few 
> seconds.  Oh well, no biggie.  Except, every so often, these I/O bottlenecks 
> start to affect other VM's and they freeze as well for a few seconds.  I 
> don't really care of one VM does so much I/O that it freezes itself 
> temporarily.  I just don't want I/O bottlenecks on one VM to affect other 
> VM's.
> 
> My questions are:
> 
> 1. Is this a problem anyone else has experienced and has it been fixed in a 
> later KVM release?
> 
> 2. I'm using the CFQ scheduler.  Would the deadline scheduler do a better 
> job?
> 
> 3. Any other suggestions to improve this problem?

Are you using filesystem backed storage for the guest images or direct
block device storage? I assume there's heavy write activity on the
guests when these hangs happen?

ext3 with ordered data mode has latency issues on fsync.