Should I defrag my Guest OS?

A very good post that answer this question,
http://blogs.vmware.com/vsphere/2011/09/should-i-defrag-my-guest-os.html#comment-203323

But in general, NO, you should not defrag the Guest OS:

1. Defragmentation also generates more I/O to the disk. This could be more of a concern to customers than any possible performance improvement that might be gained from the defrag.
2. There isn't any noticeable improvement in performance after a defragmentation of Guest OSes residing on SAN or NAS based datastores. (As VM Says)