What version of Ontap ? - sounds like the bug we encountered - I wrote it up here

http://www.vmadmin.info/2010/11/vfiler-migrate-netapp-lockup.html

Bug ID 90314 Title Heavy file deletion loads can make a filer less responsive

Basically the fix was setting these hidden options to de-prioritize volume deletion related

operations (these ops had swamped the netapp during an aborted vfiler migrated (that's another related issue))

options wafl.trunc.throttle.system.max 30
options wafl.trunc.throttle.vol.max 30
options wafl.trunc.throttle.min.size 1530
options wafl.trunc.throttle.hipri.enable off
options wafl.trunc.throttle.slice.enable off

So far, we have not seen the issue again.

internal snapshot, snapmirror, dedup operations can account for a large percentage of load and IO