Proper stride and stripe-width for RAID 50

[Andreas Dilger]

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.

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

>  These settings should work equally well whether the two RAID 5s are striped together or just appended one after the other via LVM.
> 
> Is my reasoning correct?
> 
> David
> 
> 
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Cheers, Andreas