Using su and sw when the underlying RAID changes

Using su and sw when the underlying RAID changes

[David Shaw]

Hello,

I have a question around su and sw on a RAID.  Using some RAID systems, including mdadm under Linux, it's possible to reshape the array such that the su and sw settings used at mkfs time are no longer the correct ones.  For example, lets's say I make a 2 device RAID 0, run mkfs.xfs on it, and use it for a while.  That RAID is sw=2.  Then I need more space so I add another device, reshaping it to a 3 device RAID 0.  If I did a fresh mkfs.xfs on the (now) 3 device RAID 0 I would have sw=3, but given that I don't want to lose all the contents, the existing formatting is still sw=2.

Are there any best practices (including, I suppose, "don't do that") for handling this sort of thing?  So far as I know there is no way to change su or sw for already allocated data structures.  I understand that the existing data will still be readable, albeit not optimally stored.  How bad is it to have incorrect su or sw?

Thanks for any advice,

David