Proper stride and stripe-width for RAID 50

[David Shaw]

On Jun 5, 2012, at 12:18 PM, Andreas Dilger wrote:

> On 2012-06-05, at 10:12 AM, David Shaw wrote:
>> I've been looking around, but can't seem to find an authoritative statement on setting stride and stripe-width for RAID 50 (i.e. a RAID 0 over multiple RAID 5s)
>> 
>> Based on my understanding of what stride and stripe-width set, it seems to me that it should be calculated the same as it would be if there were no multiple-level RAIDing involved.  For example, given a RAID 50 made up of two 3+1 RAID 5s striped together (so 8 disks total) with a 512k chunk size and 4k block size, the stride should be 128 (512 / 4) and the stripe-width should be 768 (stride * 6 data disks).
> 
> Strictly speaking, you only need a stripe-width of 384 (stride * 3 data
> disks) since this is the minimum read-modify-write boundary.

Ah, right.  Thanks, I was over-thinking it.

> That said, why not configure your system with RAID-6 6+2?  This gives
> better fault tolerance than RAID-5.

Yes.  I was using two RAID5 3+1s as a simple example.  One of the systems I'm working on actually has 24 drives, and I thought four RAID5 5+1s might perform better than two RAID6 10+2s (I'm going to actually test that of course).

David