Quoting Jay Soffian (jay@cimedia.com):

Okay, I've got to know. What does 'evil_twins' do?

In 5.1 and later, large file resizes are optimized with the help of an algorithm we named "evil twin". It seemed funny at the time, it was late at night, etc. The algorithm involves splitting a resized file into two: the survivor and the "evil twin", the part which will be deleted. The evil twin can be deleted asynchronously (just like we do with a normal nfs delete operation) so you don't have to wait around the next time you resize that 50GB log file to 0. Just try it with some other brand of NFS server. :)

Anyway, the "evil_twins" command just tells you how many times the algorithm has been used since booting.

Paul Eastham Netapp Engineering