[Linux-cluster] GFS Performance Problem / coherency between GFS access speed and network bandwidth

[Christian Brandes]

I found out, that there must be a coherency between GFS access speed and
network bandwidth allthough i/o is going through FC to the RAID in my 
4-node cluster.

It is 4 nodes each redundantly connected by 4GB-FC to the two
controllers of a FC-to-FC RAID System.
Devices are accessed through multipath-tools' /dev/mapper devices on the
primary path.
Each node has 1 100BaseT and 2 1000BaseTX network interfaces.

Before installing the cluster I did some performance tests such as
writing from /dev/zero to the RAID containing an XFS file system and now
with a GFS on it.

Here are some numbers:

FS      heartbeat-network      access-speed     latency
----------------------------------------------------------
GFS     10MBit/s               50MB/s           very high
GFS     100MBit/s              66MB/s           high
GFS     1000MBit/s             100MB/s          low
XFS                            260MB/s          none

What does GFS do over network when it is accessed by a node.
Does dlm use such a high network bandwith before the first bit is
written through FC to the RAID?

I had a VM on VMWare Server running on GFS that was interrupted every 10
seconds for one ore two seconds.
Now with 1000MBit/s it is not interrupted any more. But do not want to
waste more than 60% of our RAID's performance.

What can I do to speed up GFS?

Hope you have an idea.

Best regards

	Christian