When I think about Virtualization on NFS I jump right on the storage efficiency and density band wagon. Don't get me wrong, NetApp's blocks story with virtualization is awesome too. However, when someone asks me (Technical Evangelist for NAS<-- 🙂) how many VM's can be stored on a single NFS mount point the answer is astonishing -- How many does your hypervisor vendor support?
At this point it's about 256 <--- look familiar? 2^8; good ole 8-bit there. So now you have 256 Virtual machines running off the same export. Now imagine if those are Virtual Desktops; that's quite a bit of density vs blocks. SCSI limits to about 256 LUNs which is on par; but why do you need seperate luns? If You need seperate luns if you want to move the VM from one physical server to another. In that case the LUN must move - so having a lot of VMs per LUN can constrict your virtual data center management.
Beyond density I get into the futures of NFS, which include NFSv4. The big deal with NFSv4 is delegations and lock management. Not that NFSv3 has any issues, but NFSv4 read/write file delegations would give hypervisor servers more local control over the data and file locking is more resolute. Further, NFSv4 supports the notion of referrals which would allow a NFS server node to redirect a NFs client to a less burdened NFS node in a clustered storage environment, such as Data ONTAP GX. All for naught, as the primary NFS version is version 3, most hypervisor servers don't support NFSv4 and Data ONTAP GX doesn't support NFSv4 today 😞
However, in the not to distant future hypervisor vendors may chose to support NFSv4. Data ONTAP 7.3+ supports it and Data ONTAP 8 cluster mode storage will support NFSv4 - combining it with hypervisor support could be truly beneficial. Then, just over the horizon is parallel NFS (NFSv4.1) which gives rise to more predictable performance at the NFS client (hypervisor server). The predictability arises out of the support for parallel data servers, since those data servers can be clustered you can create NFS volumes across at least two nodes that are also clustered - giving you four systems to support parallel reads and writes. If one node needs to be upgraded at least three nodes would still be on line servicing requests (for that volume). Finally, during all this parallel data management, if the workload is primarily read then you can introduce FlexCache into the mix. It supports data center scale-out and remote-office/branch-office accessibility by NFSv3 clients today. In the world of ONTAP 8 - FlexCache will get even better - shhh 🙂
So if you start using NFS for all the reasons above; you'll be ready to take advantage of the system enhancements coming with NFSv4 and NFSv4.1.
Perhaps you can encourage your hyperviser vendor to do the same 🙂
~~Joshua