Impact of Nutanix VSS Hardware Support

When 4.6 was released I wrote about how the newly added VSS support with Nutanix Guest Tools (NGT) was the gem of the release. It was fairly big compliment considering some of the important updates that were in the release like cross hypervisor DR and another giant leap in performance.

I finally set some time aside to test the impact of taking a application consistent snapshot with VMware Tools vs the Nutanix VSS Hardware Support.

vmware-vss-qWhen an application consistent snapshot workflow without NGT on ESXi, we take an ESXi snapshot so VMware tools can be used to quiesce the file system. Every time we take an ESXi snapshot, it results in creation of delta disks, During this process ESXi “stuns” the VM to remap virtual disks to these delta files. The amount of stun depends on the number of virtual disks that are attached to the VM and speed in which the delta disks can be created (capability of the underlying storage to process NFS meta-data update operations + releasing/creating/acquiring lock files for all the virtual disks). In this time, the VM is totally unresponsive. No application will run inside the VM, and pings to the VM will fail.

We then delete the snapshot (after backing up the files via hardware snap on the Nutanix side) which results in another set of stuns (deleting a snapshot causes two stuns, one fixed time stun + another stun based on the number of virtual disks). This essentially means that we are causing two or three stuns in rapid succession. These stuns cause meta-data updates in addition to the flushing of data during the VSS snapshot operations.

Customers have reported in set of VMs running Microsoft clustering, these VMs can be voted out due to heartbeat failure. VMware gives customer guidance on increasing timers if your using Microsoft clustering to get around this situation.

To test this out I used HammerDB with a SQL 2014 running on Windows 2012R2. The tests were run on ESXi 6.0 with hardware version 11.


VMware Tools with VSS based Snapshot
I was going to try to stitch the images together because of the time it took but decided to leave as is.


The total process took ~4 minutes.

NGT with VSS Hardware Support based Snapshot
NGT based VSS snapshots don’t cause VM stuns. The application will be stunned temporarily within Windows to flush the data, but pings and other things should work.


The total process took ~1 minute.


NGT with VSS hardware support is the Belle of the Ball! While there is no fixed number to explain the max stun times. It depends on how heavy the workload is but what we can see is the effect of not using NGT for application consistent snapshot and it’s pretty big. The collapsing of ESXi snapshots cause additional load and should be avoided if possible. NGT offers hypervisor agnostic approach and currently works with AHV as well.

Note: Hypervisor snapshot consolidation is better in ESXi 6 than ESXi 5.5.

Thanks to Karthik Chandrasekaran and Manan Shah for all their hard work and contribution to this blog post.


  1. “Hypervisor snapshot consolidation is better in ESXi 6 than ESXi 5.5.”
    Is there any comparison article? Please let me know please. Thanks.

    • dlessner says:

      Hi, I don’t remember the article but it was mentioned apart of the ESxi 6 release from VMware.

  2. Did you test this with VMFS and NFS? I’m guessing it’s with VMFS and with NFS, the performance will be better because of the file locking mechanism.


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