Java eh? I guess that means memory hog? Mostly Opterons you say...so...what are the none Opterons running? Have you noticed swapping on non-Opteron boxes? Opterons have automatic memory handling advantages without tuning under the Linux kernel. As for I/O, it depends on what kind of patterns you have and what kind of behaviour do you require. Do you have lots of files in a single directory? Do you require sync? What is your RAID for? I have had plenty experience in squeezing performance out of 2.6 kernels but not that much with Centos 4.x boxes except for discovering that Centos 4.x kernels don't cut it. As to why I found out later but that only applies to Intel-based processors apparently...that is the bad vm patch Redhat applied to their kernels of which the removal will come in Update 4. RHEL 4 limiting themselves to ext3 leads to there being less to try... Anyway, I am very into optimization since that was how I did not have ask for extra machines for the mail server clusters that I use to watch over in my previous job.