Spares can fill in for drives smaller than the spare, so technically as long as your spare is the same size as your largest drive, you're okay. So if you have 18, 36 and 72 GB drives and a 72 GB spare, and you lose an 18 GB drive, then the 72 GB spare will take over for it.
On the downside, that disk is now a permanent member of the RAID set and you've lost all that extra space. The only way I know of to reclaim it would be to replace the failed 18 GB drive, then do a disk fail on the 72 GB drive, let it rebuild onto the 18 GB drive, then disk erase the 72 and put it back in as a spare. Big PITA.
Generally, I prefer to keep a spare for each disk size. Life is easier that way.
On Thu, 8 Aug 2002, Deron Johnson wrote:
Also, if you have multiple disks sizes in a filer, you will want one hot spare for each disk size.