Success in improving RAID speeds

After a long and hard trial process, I have increased the speed of my RAID5 array on an IBM ServeRAID-6M to a very acceptable level

Before I worked on this, guests on a 5 disk RAID5 were getting 5MB/s and after-wards I am getting the following Speeds

• Windows 2003, 12GB, Read tests with HD_speed are 92 MB/s
• Ubuntu 9.04 server, 8GB Read+Write tests, using dd are 32 MB/s

My second RAID was only 3 disks because I had a problem with 2 of the disks. Here are my suggestions for building a RAID5 from used parts

• Test the READ and WRITE speeds of each disk by itself. I had 1 U320 disk that was reading fine at 60 MB/s but crippled down and would only write at 5 MB/s
• Have the same interface; one disk was a U160, and it could only read at 50 MB/s
• Do not include any disks that are considerably slower than others, it will slow down the whole array.
• Not sure this is true with all Array controllers, but make sure that both the disks and the array are set to write back, not write through. Also the setting for the disks can only be change while they are not in an array, and cannot be change once they are built into an array

These are my preferences, and not sure they will affect anything.

• When building the array use the largest stripe size. The ServeRAID-6M goes up to 64K
• When formatting the VMFS Datastore choose a larger block size like 4MB or 8MB.