CentOS - Jul 2010 - Server rebooted, log message "kernel: Keepalive, Event:mdseve_cookie, client ping timeout"

Hi.   Our CentOS 5.4 server just rebooted sponteneously while doing heavy I/O.

There is a message in /var/log/messages just prior to reboot,

        kernel: Keepalive, Event:mdseve_cookie, client ping timeout

Google does not turn up anything.   What does this mean and how can I prevent
it from happening again, please?

Best,
-at

P.S.  I do have two kernel dumps from two reboots this morning.  But I
don't know how to read them or what to look for.  Would appreciate any
RTFM pointers.