What is the different between Redundancy Factor and Replication Factor?

Redundancy Factor (aka FT – Fault Tolerance)  in the simplest terms, is the number of components that a Nutanix cluster can withstand at any time +1. These components include disks, NIC’s, and nodes. For example, in a two block environment and the default Redundancy Factor of 2, you can lose a disk, NIC or node and still maintain no data loss.

This is not to be confused with Replication Factor, which is the number of times that any piece of data is duplicated on the system. This is directly tied to the Redundancy Factor setting. For example, if you have Redundancy Factor of 2 on the cluster, the only option for Replication Factor on Storage Containers is 2. A Redundancy Factor of 3 on the cluster will allow for a Replication Factor of 2 or 3 for Storage Containers.

NOTE #1: You can have a container with RF=2 and another container with RF=3 configured on the same Nutanix cluster.

NOTE #2: Changing container replication Factor from RF=2 to RF=3, consumes more storage space on Nutanix cluster (because systems have to keep 3 copies of the VM data which are on containers with RF=3).

Redundancy Factor (FT)

By default, Nutanix clusters have redundancy factor 2, which means they can tolerate the failure of a single drive, NIC or node. The larger the cluster, the more likely it is to experience multiple failures. Without redundancy factor 3, multiple failures cause cluster unavailability until the failures are repaired.

Redundancy Factor 3 (FT3)

Redundancy factor 3 has the following requirements:

• A cluster must have at least five nodes for redundancy factor 3 to be enabled.
• For guest VMs to tolerate the simultaneous failure of two nodes or drives in different blocks, the data must be stored on storage containers with replication factor 3.
• Controller VM must be configured with enough memory to support redundancy factor 3.

Changing the Redundancy Factor from 2 to 3

NOTE: Setting the Redundancy Factor can only be configured before the cluster is created.

1. To increase the cluster from redundancy factor 2 to redundancy factor 3, log on to any Controller VM in the cluster through SSH and start the nCLI.
2. To view the cluster redundancy factor state:

ncli> cluster get-redundancy-state

3. Output similar to the following shows redundancy factor 2.

   Current Redundancy Factor : 2
   Desired Redundancy Factor : 2
   Redundancy Factor Status :
   kCassandraPrepareDone=true;kZookeeperPrepareDone=true
   

4. Set the cluster to redundancy factor 3.

   ncli> cluster set-redundancy-state desired-redundancy-factor=3

5. Output similar to the following is displayed.

   Current Redundancy Factor : 2
   Desired Redundancy Factor : 3
   Redundancy Factor Status : –

6. The nCLI output might take several minutes to update the redundancy factor.
7. Verify that the redundancy factor is now 3.

ncli> cluster get-redundancy-state

8. Output similar to the following shows redundancy factor 3.

Current Redundancy Factor : 3
Desired Redundancy Factor : 3
Redundancy Factor Status : kCassandraPrepareDone=true;kZookeeperPrepareDone=true