Explain difference in SQLIO numbers for RAID 0 versus RAID 5 over 6 disks

Question

When using the SQLIO benchmark tool on a 4-core Dell server with 6 15k 450GB (fast) drives, RAID 0, we found the max throughput was 2MB per second. But when configured as RAID 5, we get 30 MB per second.

It seems that the RAID controller, Dell Perc 5i integrated controller, is maxing out the throughput per disk. With RAID 5, I expect to get a bump due to stripping, but not a 15x difference.

Like good programmers, we suspect the hardware , but we could be missing something.

This is predominately write traffic.

Answer 1

RAID 0 write speeds should be significantly faster than RAID 5. RAID 0 stripes across all the drives so parts of the data you are writing will end up spread across the 6 disks.

RAID 5 also spreads the data across the drives but actually writes extra parity data so it can store the parity data in case one of the drives goes bad.

30 MB seems a little low on write speed to me for RAID5 in that setup but isn't terrible. 2MB per second just plain terrible, especially for RAID 0. I don't think I have enough info to tell you if it is hardware or not but I can tell you that something is wrong with this.

Answer 2

In your update, it doesn't make any sense that you configured each disk into a RAID 0.

RAID 0 is striped too, so you need more than one disk.