VMware Embraces NFS 4.1, Supports Multipathing and Kerberos Authentication

Welcome to the vSphere 6.0 ZOMG Series, focused on the tech goodies baked into 6.0. You can click me to return to the directory, or choose a different deep dive from the menu at the top.
Welcome to the vSphere 6.0 ZOMG Series, focused on the tech goodies baked into 6.0. You can click me to return to the directory, or choose a different deep dive from the menu at the top.

It’s no secret that I’m a fan of using file level storage for virtual machines; block storage makes less and less sense for virtualized environments, especially when I see all the trickery that must occur for a block device to be VM-aware. For vSphere, the desire to use file based storage meant selecting one protocol: NFS version 3. I was always a bit sad that version 4 or 4.1 was not added to the hypervisor because of all the great improvements that were baked in.

In a move that I still find to be a bit shocking, vSphere 6.0 has added support for NFS version 4.1. This includes a great many new features and improvements:

  • Authentication with Kerberos
  • In-band, mandatory, and stateful server side locking
  • Session trunking (true NFS multipathing)
  • Greater error recovery

Be aware that in the release, datastores that are mounted using NFS version 4.1 cannot be combined with SDRS, SIOC, SRM, or Virtual Volumes. Additionally, NFS 4.1 datastores are not yet compatible with VAAI-NAS (hardware acceleration) as per the storage guide:

[NFS 4.1] Does not support Hardware Acceleration. As a result, virtual disks created on NFS 4.1 datastores are always thin-provisioned.

This is a deal breaker – this needs to be fixed before I can see folks wanting to go wild with NFS 4.1. The other features work just fine: vMotion, HA, FT, DRS, and Host Profiles.

pNFS is not Session Trunking

I’m seeing a lot of weird discussions on pNFS (Parallel NFS) versus Session Trunking.

To be clear, Session Trunking is similar to MPIO with iSCSI or Fiber Channel – you create multiple paths (sessions) to the NAS array and distribute load across those sessions. It’s up to the storage array vendor to determine if that can be done across NAS controllers or not based on their architecture: active / active or active / passive controllers. Session Trunking is awesome because it trivializes 99% of the work required to actively use multiple network links to an NFS datastore.

pNFS is a unicorn and is barely defined in the NFS 4.1 standard. vSphere 6 does not support pNFS, and neither does most anything else in the world. Here’s a high level diagram I used to describe the topology while at the Melbourne VMUG Conference back in early 2014.


It’s up to the storage vendor to figure out the metadata server, the control plane, and the data protocol used on the storage arrays – that’s not part of the NFS 4.1 specifications.

NFS Versions and Guidelines

If you’re looking to use NFS version 4.1 today, you’ll need to make sure that all ESXi hosts are mounting the datastore using 4.1. You can’t mix protocol versions for the same datastore across hosts because they use entirely different locking methods. I was pondering this and came up with a few migration ideas:

  1. Mount a new volume using NFS 4.1 and Storage vMotion workloads to it.
  2. Take an outage and switch all of the hosts over to 4.1

Both of these ideas kinda suck. It would be much nicer if there were a migration path in place that allows both locking protocols to work in tandem during the migration.

VMware has also published some helpful NAS guidelines in their storage guide. Specifically:

  1. To use NFS 4.1, upgrade your vSphere environment to version 6.0. You cannot mount an NFS 4.1 datastore to hosts that do not support version 4.1.
  2. You cannot use different NFS versions to mount the same datastore. NFS 3 and NFS 4.1 clients do not use the same locking protocol. As a result, accessing the same virtual disks from two incompatible clients might result in incorrect behavior and cause data corruption.
  3. NFS 3 and NFS 4.1 datastores can coexist on the same host.
  4. vSphere does not support datastore upgrades from NFS version 3 to version 4.1.
  5. When you mount the same NFS 3 volume on different hosts, make sure that the server and folder names are identical across the hosts. If the names do not match, the hosts see the same NFS version 3 volume as two different datastores. This error might result in a failure of such features as vMotion. An example of such discrepancy is entering filer as the server name on one host and filer.domain.com on the other. This guideline does not apply to NFS version 4.1.
  6. If you use non-ASCII characters to name datastores and virtual machines, make sure that the underlying NFS server offers internationalization support. If the server does not support international characters, use only ASCII characters, or unpredictable failures might occur.

Preparing for Kerberos Authentication

If you’d like to use Kerberos authentication, which isn’t a requirement, here’s how you set it up. First, make sure you have a reliable time source configured on the host to avoid a time drift between the Kerberos server (AD DC) and your host. A skew of 5 minutes will typically result in failure.


Next, add your host to the AD domain. This is the same process it has always been.


Finally, give the host an AD service account to use for authenticating to the NAS. You can only configure one account per host and VMware recommends using the same account across all hosts.


So long as the NFS volume can be mounted by that service account and supports RPC header signing (header auth), along with DES (specifically DES-CBC-MD5), you’re good to go. Apparently AES-HMAC was not used due to lack of vendor support, which is why such an old crypto is used.

Mounting an NFS 4.1 Share

Here’s the workflow to mount an NFS share using protocol version 4.1. First, add a datastore and select NFS 4.1.


Configure the datastore name, path (folder), and server address. If you want to enable session trunking for multiple paths, enter multiple IP addresses that are available on your NAS. Most arrays will allow you to configure virtual interfaces (VIFs) or virtual IPs (VIPs). Below I’ve configured and .112 as VIFs on a NAS running NFS 4.1.


Optionally, enable Kerberos auth. If you don’t check this box, the host will use AUTH_SYS.


When you fire over a mount request, the host will log this entry to vmkernel.log:

NFS41_VSIMountSet:402: Mount server:,, port: 2049, path: Treasure, label: Treasure, security: 2 user: [email protected], options: <none>

Practice taught me that incorrectly configuring the volume permissions resulted in an access denied error, or mounting the storage as read only. Here’s a sample connection to an improperly configured Windows Server providing storage via NFS.

WARNING: NFS41: NFS41FSCompleteMount:3601: RECLAIM_COMPLETE FS failed: Failure; forcing read-only operation

The new vSphere Web Client will show you specifically what version of NFS is being used to mount the datastore.


Link Failure

I deleted the address from the NAS server to simulate a link failure. As expected, the hypervisor got a little upset.

WARNING: SunRPC: 3947: fail all pending calls for client 0x4302f12611a0 (socket half closed)
WARNING: SunRPC: 3947: fail all pending calls for client 0x4302f125f9a0 (socket half closed)
WARNING: SunRPC: 3947: fail all pending calls for client 0x4302f125f9a0 (socket disconnected)
WARNING: SunRPC: 3947: fail all pending calls for client 0x4302f125f9a0 (socket disconnected)

But the NFS share remained mounted and the contents were still viewable. Look, it’s my hidden treasure!


Support for NFS 4.1

I actually had a bit of difficulty finding something for my lab that even supports 4.1. Supporting NFS 4.0 isn’t enough.

In case you’re curious, here’s a list of arrays that do not support NFS 4.1:

  • Synology DSM 5.1
  • FreeNAS
  • OpenFiler
  • Nexenta’s NexentaStor Community Edition
  • NetApp’s OnTAP Simulator (8.2.1 7-mode)

In the end, I used a Windows Server 2012 VM with the NFS Server feature installed. Ironic, eh?

Perhaps you’re curious what happens when you try to mount an NFS volume using 4.1 to a NAS that does not support that protocol? It’s simple: you’ll get a timeout failure. Not terribly helpful, is it?


Dive into vmkernel.log to find more details. The following log detail is quite helpful!

WARNING: NFS41: NFS41ExidNFSProcess:2008: Server doesn't support the NFS 4.1 protocol


It would be nice if that error rolled up into the mount task.


I want to spend much more time experimenting with failure scenarios and really seeing how well the session trunking configuration is able to balance load across the physical links. With that said, I’m really excited to see NFS 4.1 come to ESXi. NFS 3 is extremely old and clunky but also quite reliable and able to push some great performance numbers. My hope is that vendors will now have much more of an incentive to add NFS 4.1 support to their arrays and future iterations of the hypervisor will continue to improve upon the IETF standard (RFC 5661).