. Can anyone fill me in on the use of this _wac_ command , what it does,
It's "wack" and it's similar to fsck for UNIX or scandisk for NT. It corrects any errors to the filesystem that may have been introduced due to unusual crashes or hardware failures.
how often it needs to be done,
Whenever the filesystem indicates corruption. This usually happens if you have a crash and/or the filer does not boot up properly and Netapp's tech support tells you to run wack. For some customers, they never have to run wack. For others, they may run into a series of bugs that require them to run it every couple of months.
Keep in mind that you won't need to run it every time your filer crashes or you have a power outage, unlike your UNIX and Windows systems, but only on rare occasions.
and whehter or not the filer requires downtime to do it?
Yes, it does. (Although that may be changing.)
Bruce