If I add a disk to a volume, what will it do to performance, and for how long? I don't mean long-range gains from having the data spread over more spindles, I mean what immediate impact will it have on current NFS clients. Will it just be a temporary increase in CPU as it spreads the data or will it freeze for some period of time, and if so for how long?
Adding a disk into a volume will cause increased CPU and I/O load for < 10 minutes while data is copied over. It won't freeze the system or do anything disasterous like that.
I think this is right, of course there is the possibility that this only works with actual failed disks, but...
if you are really paranoid about the possible impact adding a new disk to an existing volume will have on your filer you can play with the raid.reconstruct_speed option. crank it down and you should see very minimal impact to the system, but it will take aproximately forever to finish. crank it all the way up and clients may start having issues accessing the filer. i've found that staying around 6 seems to speed things up a little ( maybe it's the placebo effect ), and not impact performance terribly... default should be 4 which on most of my filers is about right.
-s