Two node clusters

  • 7 December 2019
  • 7 replies

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A traditional Nutanix cluster requires a minimum of three nodes, but Nutanix also offers the option of a two-node cluster for ROBO implementations and other situations that require a lower cost yet high resiliency option. Unlike a one-node cluster (see Single-Node Clusters), a two-node cluster can still provide many of the resiliency features of a three-node cluster. This is possible by adding an external Witness VM in a separate failure domain to the configuration (see Configuring a Witness (two-node cluster)). Nevertheless, there are some restrictions when employing a two-node cluster. The following links will provide you guide lines and information abut configuring the two node clusters:

Ask any questions to clarify any concerns about the the two node clusters.

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How can I create a 2 node cluster?

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@hienle You can review the links above. Below is a snip of what you will see under the title “Two node cluster guidelines”


A two-node cluster is configured and upgraded like a regular (three-node or more) cluster in most ways, but note the following for a two-node cluster:

  • Size your implementation for N + 1 so that in the event of a node loss (50% loss of resources) the remaining node will have sufficient resources to allow the cluster to continue functioning.
  • There is a heartbeat check (ping) between the nodes every two seconds. If a successful ping does not occur within 10 seconds (5 consecutive failed tries), a failover is initiated (see Failure and Recovery Scenarios). When the cluster recovers, it must remain in healthy status for at least 15 minutes before it will failback.
  • The upgrade process in a two-node cluster may take longer than the usual process because of the additional step of syncing data while transitioning between single and two node state. Nevertheless, the cluster remains operational during upgrade.
  • Witness VM considerations
    • A Witness VM for two-node clusters requires a minimum of 2 vCPUs, 6 GBs of memory, and 25 GBs of storage.





hope this helps a little or ask more questions.

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Be VERY careful when considering a two node environment.  We’ve run into a number of caveats one of which I haven’t seen documented.  When going through maintenance tasks (AOS upgrade, firmware upgrades, etc) we’ve seen what looks to be the data uncompressing when transitioning from a two node to single node cluster.  I’ve attached a screenshot showing the storage usage of our container as it transitioned between two and one node clusters.


I have a ticket opened with Nutanix to get clarification, but from what we see enabling compression is not a good idea when using a two node deployment.  After having our deployment live for a few months I feel like it’s not ready for prime time yet.  There are a lot of caveats vs. a 3+ node cluster and for what what you pay I feel like other HCI players do a much better job.

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I wanted to provide an update on some testing I’ve done in our two node environment (running ESXi).  From what we were seeing during previous maintenance tasks there was an increase of usage during two node → one node transition so I wanted to test this further.  We have our container configured with “Advertised Capacity” at the usage threshold recommended by Nutanix in a two node cluster (40% which takes into account a node failure).  I then filled the datastore to ~95% capacity and applied AOS (coming from 5.11.2).  During the upgrade process we did reach max capacity (the Advertised Capacity size) briefly, but it didn’t affect the VMs running.  So I can’t really explain what’s happening behind the scenes, but I do feel better about leaving compression enabled.



Even though the two node cluster has some caveats, Nutanix providing the ROBO option licensing on a per VM basis makes this an enticing option for a very small site.  If you were to go with the full core/capacity licensing you might-as-well do a three node to get the full feature set of AOS.

I’m trying to setup a two-node cluster in my lab but receiving errors when running the cluster create command (cluster -s, create).  I looked at the documentation and believe I’m following the correct steps but hopefully someone can provide me some guidance.  Here are the steps I’ve follows:

  1. Deployed a witness VM in a separate environment and performed a basic network configuration on it.  That part appears to have worked.
  2. Installed two nodes w/o adding them to a cluster.  Adjusted the CVM resources to 6 vCPUs and 20GB of memory as recommended and powered them on
  3. SSHd into both of the CVMs and was able to ping the other CVM to confirm network connectivity
  4. Ran the cluster create command above

I see services starting up until it fails with the following error:  

Apr 27 21:16:23 5846302e2744 httpd: [Mon Apr 27 21:16:23.939426 2020] [proxy_html:notice] [pid 3406:tid 140385313859712] AH01425: I18n support in mod_proxy_html requires mod_xml2enc. Without it, non-ASCII characters in proxied pages are likely to display incorrectly.
Apr 27 21:16:23 5846302e2744 httpd: AH00526: Syntax error on line 16 of /etc/httpd/conf.d/autoindex.conf:
Apr 27 21:16:23 5846302e2744 httpd: Invalid command 'IndexOptions', perhaps misspelled or defined by a module not included in the server configuration
 Apr 27 21:16:23 5846302e2744 systemd: httpd.service: main process exited, code=exited, status=1/FAILURE
Apr 27 21:16:23 5846302e2744 kill: kill: cannot find process ""
Apr 27 21:16:23 5846302e2744 systemd: httpd.service: control process exited, code=exited status=1
Apr 27 21:16:23 5846302e2744 systemd: Failed to start The Apache HTTP Server.
Apr 27 21:16:23 5846302e2744 systemd: Unit httpd.service entered failed state.
Apr 27 21:16:23 5846302e2744 systemd: httpd.service failed.


I tried to run the command again and received the following message:

2020-04-28 20:17:43 CRITICAL cluster:994 Could not discover all nodes specified. Please make sure that the SVMs from which you wish to create the cluster are not already part of another cluster. Undiscovered ips :,

Any assistance would be greatly appreciated.


Can a two-node cluster later be converted to a three-node cluster?

No, a two-node cluster cannot be expanded later. 

See Single-Node and Two-Node Clusters FAQ (