Ext3 users - Jun 2012 - Proper stride and stripe-width for RAID 50

Hello,

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).  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

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