Running a lot of Red Hat VMs in your virtual infrastructure, on shared storage? CentOS, Scientific Linux, both versions 4 and 5, they count for these purposes; Fedora should likely be included too. Do you have the slocate (version 4.x and earlier) or mlocate (version 5.x) RPMs installed? If you’re uncertain, check using the following:
> rpm -q slocate
> rpm -q mlocate
If so, multiple RHEL VMs plus mlocate or slocate may be adding up to an array-crushing 4:02am shared storage load and latency spike for you. Before being addressed, this spike was bad enough at my place of employment (when combined with a NetApp Sunday-morning disk scrub) to cause a Windows VM to crash with I/O errors. Ouch.