A short follow up to indicate how I solved my problem : - moved all the ram files to /dev/shm - downgraded host to CentOS 4.8 (was 5.2) - Moved virtual disks to RAID1 (was RAID5) - Spread the virtual disks over various raidsets (was all on same raidset) The first element alone was not helpful. I was not able to test RAID1 vs RAID5 in isolation from 4.8 vs 5.2, which would have been nice. I might be downgrading all the other hosts to 4.8, in which case I might be able to test it in isolation. -Philip