Raid 5 scaling question?

[w0ss]

I am testing raid 5 performance on some scsi drives

I have 6 HDD's and I am trying to figure what will get me the best performance. I ran sandra and tested 3 drives raid 5 then 6 drives raid 5. The results surprised me and I ran the tests 3 times each to verify I was seeing the truth.

Going from 3 to 6 drives

1. No increase or decrease in
Buffered Reads(117MB/s), Sequential Reads(74MB/s), or Buffered Writes(110MB/s).

2. Saw an increase in
Random Reads(33MB/s to 50MB/s)

3. Decrease in
Sequential Writes(49MB/s to 36MB/s), and Random Writes(41MB/s to 30MB/s)

Is that to be expected? I would think I should see an increase in all performance or at least the same performance. Anyone have another HDD benchmarking utility.
Thanks,
W0ss

 

[lobadobadingdong]

yes it's to be expected, overall speed using raid 5 is dependant on a lot of factors. raid controller, amount of raid memory, speed of raid memory, strip size, average file size, quality of hte backplane/scsi cable, etc.

 

[w0ss]

ok thanks I wasn't sure what was normal. I upgraded the ram on the card from 32MB to 128 MB and it helped.

32MB ram
Sequential Writes(36MB/s), and Random Writes(30MB/s)

128MB ram
Sequential Writes(44MB/s), and Random Writes (39MB/s)