I've been trying to determine what the correct stripe width for a RAID-50 volume is but haven't been successful in google searches or empirical tests. The volume is built off of 4 disk spans that contain 6 disks each. If I understand it correctly, each individual span is a RAID-5 volume and the 4 spans are combined using RAID-0. However, I'm not seeing any noticable effect when I vary the stripe-width from 2-20. Suggestions?
While I realize that this is over a year old, I recently ran into the same question and came to a different conclusion and wanted to add it for posterity.
Unfortunately I think the FusionIO article mentioned here may be wrong. First, their table shows that to find the number of data bearing disks on a raid50 you take the number of "mirrored_sets" and subtract one. However it also has this text near the table:
The problem is that raid50 doesn't have any "raid 1 mirrored sets", and the whole idea just sounds kind of, funny.
The larger reason though is that I came across a more recent thread that included input from an ext guru (Andreas Dilger). The explanation on that thread made a lot more sense to me, even before considering reputation.
You can find that here, but I'm going to paste the key part below for good measure...
After digging around online, I found this line (http://kb.fusionio.com/KB/a43/filesystem-tuning.aspx) that indicated the stripe width in nested RAID levels should be based on the number of spans/groups. In my case, as I have 4 spans in RAID50, the stripe width (as used in XFS), is 3 (n-1, where n is the number of spans)