7.3.2.4 Failures to Tolerate (FTT)
The number of failures to tolerate (FTT), will determine how many copies of the object will be required. For example: FTT=1 is 2 copies, FTT=2 is 3 copies, and FTT=3 is 4 copies. This count includes the original object itself but not the witness.
In the example used in
Table 12, there are 8 objects for the virtual machine, and the following formula is used to calculate the number of components distributed across the vSAN cluster.
Formula: Objects * (copies + witness) = components
Table 13. FTT Formula Result
FTT Setting | Component copies | Formula | Total number of components required for 100 virtual machines |
1 | 2x | 8 * (2 + 1) = 24 | 2,400 |
2 | 3x | 8 * (3 + 1) = 32 | 3,200 |
3 | 4x | 8 * (4 + 1) = 40 | 4,000 |
With 100 virtual machines per host and an FTT of 1, there will be a total of 2,400 components distributed across the cluster. Even for a higher service level that would dictate FTT of 2, you can see this is well within the 9,000 per-host component limit.
In the following screenshot, the number of failures to tolerate (FTT) is set to 1, and the Horizon replica disk (VMDK) is selected. In this example, there are 2 replica components (RAID1) and a witness.
Figure 18. Witness and Replica Components
