RAID 0+1 performance with different number of drives

[Brent Boisvert]

I realize that placing only one disk per channel is extreme and not practical.

Consider a three disk RAID5 on a two-channel controller. Channel A will have two drives on it and that SCSI bus will be twice as busy as Channel B with only one drive. Channel A will have a higher bus latency. This is why controllers with more channels outperform controllers with less channels. This performance hit may be very, very small, but if the server is passing many I/O intensive requests to the disk channel (writes in particular), it could multiply the latency.

Anyway, I don't believe that with the hardware stated in the original post that you should expect to see a performance gain using more drives in RAID0+1. I agree with Leif that file size may also factor here. One large file may yeild better results that many smaller files (sequential disk access .vs random disk access)

I hope this discussion has been helpful.

Brent

[Michael Neymit]

First of all thank you everybody for your replies!

Let me try to answer some of the questions above. In the tests I conducted I was copying files of various sizes, but all of them were fairly large: 7GB, 29GB, 3GB. Based on the times recorded, I calculated the average throughput (FileSize/Seconds). I found that on RAID5 the rate of file copying was 7.5MB/sec. On RAID0+1 it was 11.8 MB/sec. Not anywhere close to the limitations of SCSI bus or disk I/O, but what is the limitation of RAID controller per channel? Unfortunatly I can not provide any more details about the RAID system I used other then what was stated in the original post -- it is 4200 Compaq controller with the default settings for both RAID5 and RAID0+1.

Thank you!