[Be Sc]

Hello everybody,

My friend is using Raid 5 on the ICH10R – thats the mother board chipset. He’s averaging 5MB/s on three hard drives…and noticable CPU draw…He just uses his computer for his small business database, no mega speed needed…

5MB/s isn’t right: sounds like one of the drives is faulty. Even if the write-back cache is not enabled, the speeds with today’s drives should be in the range of 50-120MB/s writes, 100-250MB/s reads.

That’s NOT correct!

Lets assume these disks are standard SATA Disks with about 10 – 15 ms access time (say 12.5 mean).

Performance calculation of raid5 with enabled wb cache is:
Reads: Number of Disks * IOPS per Disk
Writes: (Number of Disks * IOPS per Disk) / write penalty (which is 1 for raid lvls 0,1,10 / 5 for raid 5 / 6 for raid 6)

In Numbers
Reads: 3 * (1 sec / 12.5ms) = 240 IOPS
Writes: 3 * (1 sec / 12.5ms) / 5 = 48 IOPS

As you can see, this is less performance than with a standalone disk which can do 1sec / 12.5ms = 80 IOPS.

If yiu want to setup up your raid for maximum data protection and you either have no bbu or no usv, you have to disable cache writeback for the raidcontroller AND to disable disks cache. Just switching off the controller’s cache but leaving the disks cache on may result in data loss in case of power loss.
Not using the disk’s cache results in MAJOR performance decrease. 5MB is quite normal at this point. using even slower disks (ie 5200 rpm) results in even less performance.

One more word concerning different partitions:
Sure, different partitions may be useful. But be aware: If you use more than one partition on ONE raid volume, the performance will be shared between the volumes and you MAY decrease your performance by forcing the disks read heads to jump from onw physical section to another (causing access times, lowering IOPS performance).

The performance in MB/s can be calculated by Num IOPS * block size.
Just breaking down the ratio:
A single SATA standard drive does about 100 MB (SEQUENTIAL) read OR write (not at the same time). Sequential says: There are almost no access times because the head is already positioned correctly.
Sequential Read Rate should be around 300 MB/s. Write rate would be around 60 MB/s. With both, disk cache and controller cache empty, but enabled.

So, if you have a 50:50 read-write workload, you should choose raid 10. If you’ve got a lot of reads and just some writes you should choose raid 5.

Best regards

Benny