Strange disk performance issue

[Frank <frank@anotheria.net>]

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?

Regards
Frank