Dynamox is correct; the prefetch option is for best performance.  We've recently tweaked our guidelines on how to set VMware to work off of NFS exports.  This document is available on EMC.com and Powerlink.  H5536-vmware-esx-srvr-using-emc-celerra-stor-sys-wp (3).pdf  You can find White Papers at this location on Powerlink:  Home > Support > Technical Documentation and Advisories > White Papers > All White Papers
 Our Celerra (DART 5.6.44-408) is set up with nfs exports for VMWARE guests. Recently, during a datamover failover we saw about half of our vmware guests hang or crash. EMC experts are telling us that one of the things we can do to help prevent this sort of thing is to use the noprefetch option for the mounted filesystem(s). The doc from the EMC provided vmware best practices pdf looks like this:  Use this procedure to disable the prefetch read mechanism for a file system. From the Control Station command line, or the CLI in Celerra Manager, issue the following command to disable the prefetch read mechanism for a file system: $ server_mount    where:  = name of the specified Data Mover   = specifies mount options, separated by commas   = name of the file system   = path to mount point for the specified Data Mover  Example: $ server_mount server_3 -option rw,noprefetch ufs1 /ufs1   We're wondering if anyone has any experience with the noprefetch option as a means to prevent vmware guest failures during datamover failovers? Do we have to unmount and then remount with the server_mount command to implement this change? Thanks!   
vmware best practices with Celerra nfs mounts