Re: Strange disk performance issue

[Stan Hoeppner <stan@hardwarefreak.com>]

On 12/13/2011 3:24 AM, Frank wrote:
> 
> Hi list,
> hoping for help, I'm experiencing a strange performance issue on debian
> squeeze. The host is running 2.6.32-5-amd64 on a Xen enterprise cluster,
> filesystem lies on a NetApp storage mounted using ISCSI. Other virtual
> hosts on that cluster does not have the same performance problems.
> 
> As soon as I start to copy i.e. a 2GB file local or to a remote host,
> the iowait is rising and the host feels jerking.
> 
> sar reports %iowait > 60
> 
> Activating block_dump for a minute (echo 1 > /proc/sys/vm/block_dump)
> shows me that the process causing the iowait is jbd2 which is part of
> the ext4 filesystem.
> 
> Any ideas howto tweak the fs for better performance?

Ext4 isn't the source of the problem but simply the messenger in this
case.  Try the deadline and noop elevators and see if that helps.  The
default CFQ elevator is known to suck.  Also, post the block layer
device driver you're using to access the virtual disk.

-- 
Stan