RAID 5 is terrible with 4K random IO. I know guys who measured >1 million IOPS with MD RAID 0 and then switched to MD RAID 5 only to have performance drop from

RAID 5 is terrible with 4K random IO. I know guys who measured >1 million IOPS with MD RAID 0 and then switched to MD RAID 5 only to have performance drop from read-modify-write overhead. They could not believe that RAID 5 was the cause.