It greatly affects performance, so only run it on the weekend. You don't have to unmount anything. I didn't really realize this until I started thinking about the Bit Error Rate/Nonrecoverable Read Errors per Bits Read ratings of different hard drives. I ran the "Volume Set Functions->Check Volume Set" RAID scrubbing utility on a couple of Areca RAID systems we have. They are 10-drive RAID 6 raid sets (8+2), so rebuilding is usually pretty safe, but I figured I should try it anyway. No errors were found. Either that means that there were no errors found on all the drives or that the verify operation was able to correct any data corruption. Both of these systems are about two years old.

The Areca RAID manual suggests running the volume set verify once a week. In the older non-SAS RAID cards, you have to manually do this check, but in the 1680 series SAS RAID cards, there's a new option under "Volume Set Functions" to "Schedule Volume Check." I haven't started this on our systems in the field that have this newer card, yet.

The only thing about verifying so often is that it might wear out your disks a little faster than otherwise, although your data is safer. On my 10-drive RAID 6 sets with 500GB Seagate Barracudas, verifying took about 3 hours with about 500MB/s of verifying (including the parity data), which is about 50MB/s per drive, about as fast as is realistic for these drives to go sustained.