Nutanix cluster requires three nodes minimum but Nutanix also offers the option of two-node cluster. By configuring external witness VM in a separate failure domain to the configuration provides resiliency features of a three-node cluster.
Requirements:
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Controller VM minimum requirements: 6 vCPU and 20 GB memory
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Replication factor (RF): RF2 spanned over two nodes and RF4 for metadata on SSDs over two nodes. RF4 for metadata helps during a node failure scenario to quickly transition the healthy node to run in single-node mode with the metadata remaining disk fault tolerant. (Metadata in a two-node cluster is typically small, so the storage need for four copies is modest.)
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Drive failure effects: one node + one SSD failure (on other node) = read-only mode.
Guidelines:
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Two-node clusters are supported only on a select set of hardware models. See KB 5943 for information about supported models. supported model
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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 states. Nevertheless, the cluster remains operational during upgrade.
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Use external DNS and NTP servers. Nutanix recommends that you host the DNS and NTP servers on a different cluster.
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Node removal is not supported, but node replacement is supported (where the node remains part of the cluster). When replacing a node, the cluster remains active but will transition to standalone mode for a brief period of time.
Configure a witness
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To configure a Witness through the Configure Witness option, do the following:
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Get the credentials of the target Witness VM (IP address, user name, and password).
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Click the gear icon in the main menu and then select Configure Witness in the Settings page (see Settings Menu).
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In the Configure Witness window, enter the Witness VM IP address, user name, and password in the indicated fields, and then click the Save button.
For more details please refer to following article