My local datastore on my ESXi 4.1 server has less than 10GB left, so I purchased an IOMega/EMC ix2-200 NAS. My plan is to cold-migrate one of the VM's (CentOS 5.5/CPanel) to the NAS to free up some space. I installed the VMKernel driver and can see the NAS from ESXi.
As an experiment before I work on the live VM, I created a CentOS 5.5 VM with 384MB memory and an 8GB thin-provisioned disk. Once that was created and running, I shut it down and used the "Browse Datastore" function to browse and copy/paste the VM from the local datastore to the NAS.
The result was that it took 2 hours and 20 minutes to complete! The live VM would certainly take longer as it has a 256GB thin-provisioned disk consuming 13GB. The VM did happily fire up and run once I reconfigured the VM drive settings. I just don't have the maintenance window required to move the live VM, so either I'm doing something wrong that I could correct or I need to schedule a reaaaaaallllllyyyy long maintenance window.
The VM host is an IBM Bladecenter HS21 blade with dual Xeon 5160 processors and 10GB memory. Local datastore is 73GB SAS mirrored drives. NAS datastore is 2x1TB mirrored drives connected via gigabit ethernet. Connected to the Bladecenter switch, I can move a 4GB ISO file from my laptop to the NAS in 3 minutes. This model NAS regularly shows 60+ MB per second when tested by Tom's Hardware, CNET, etc.
Lastly, I'll add that an earlier attempt at cold migration was done with the VMware converter and that move took 15 minutes or so. However, the result wasn't usable as it presented a different ethernet interface to the VM, causing the IP configuration to be wiped out. That may be a recoverable situation, but with all of CPanel's bindings I don't want to take the chance.