How to use virtual machine storage policies to define the number of fault tolerances
FTT, or Fault Tolerance Tolerance, defines the number of failures a vSAN cluster can sustain without affecting the availability of virtual machines. But how exactly do you use virtual machine storage policies to set the level of fault tolerance?
The concept of fault tolerance in vSAN allows administrators to configure redundancy across the entire cluster. While the typical three-node setup is well-known—comprising two data nodes and one witness node for certain use cases—many may not be aware of how vSAN calculates the number of failures it can handle.
Like RAID 5 hardware, vSAN can tolerate up to three hardware component failures in a cluster. This is determined by the VM storage policy, which sets the "number of failures allowed." The FTT feature plays a key role here, defining how many failures the cluster can withstand without compromising data integrity or VM availability.
To support n failures, the vSAN cluster must have at least 2n + 1 ESXi hosts. This ensures that even with multiple failures, the system remains operational. The minimum requirement is three hosts, but as the FTT increases, so does the number of required hosts.
Figure A. Example of the failure tolerance formula
When creating storage policies, the focus is on what the cluster can tolerate. Different virtual machines might require different levels of fault tolerance. For instance, in a six-host cluster, one VM could be configured with FTT = 1 (allowing one failure), while another might need FTT = 2 (supporting two failures).
With FTT = 2, the system creates three copies of the data. Earlier versions of vSAN required more resources for this level of protection, but starting from version 6.6, erasure coding was introduced, enabling RAID 5 and RAID 6 protection, which significantly reduces storage overhead.
How to create a virtual machine storage policy
To create or edit a storage policy, log into the vSphere Web Client and navigate to Home > Virtual Machine Storage Policy > New.
As shown in Figure B, under Rule Set 1, you can choose between RAID-1 (mirror) or RAID-5/6 (erasure coding) as your fault tolerance method. However, note that RAID 5 and RAID 6 features are only available with a vSAN Enterprise license. These options are not supported in the Standard or Advanced editions.
Figure B. Choosing the fault tolerance method in the storage policy
In conclusion, VM storage policies and FTT are essential components of VMware's vSAN solution. They offer a flexible way to manage fault tolerance at the VM or disk level. By using erasure coding, higher FTT settings can be implemented with less storage overhead. Remember, the higher the FTT, the more resources are dedicated to recovery and redundancy, ensuring continuous availability and data protection.
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