go here http://blog.scottlowe.org/2010/01/31/emc-celerra-optimizations-for-vmware-on-nfs/
Turn on the uncached write mechanism for NFS file systems used as VMware datastores. This can have a significant performance improvement for VMDKs on NFS but isn’t the default setting. From the Control Station, you can use this command to turn on the uncached write mechanism:
server_mount <data mover name> -option <options>,uncached <file system name> <mount point>
Be sure to review pages 99 through 101 of the VMware on Celerra best practices document for more information on the uncached write mechanism and any considerations for its use.