FTT, or Fault Tolerance Tolerance, defines the number of failures a vSAN cluster can withstand without affecting data availability. To configure this, administrators use virtual machine storage policies to set the desired level of fault tolerance. These policies are crucial in determining how the cluster handles hardware or network failures while maintaining performance and data integrity.
vSAN is designed with redundancy in mind, similar to RAID configurations. It allows for up to three hardware component failures, depending on the cluster setup. The key to achieving this is through the use of VM storage policies that define how many failures the system can tolerate. This is where FTT comes into play—setting the threshold for acceptable disruptions.
The formula used by vSAN to determine the required number of hosts for a certain FTT value is straightforward: to tolerate n failures, you need at least 2n + 1 ESXi hosts. For example, if you want to tolerate one failure (FTT = 1), you need a minimum of three hosts. If you aim for two failures (FTT = 2), you’ll need five hosts, and so on.
This approach ensures that even if multiple components fail, the cluster can continue operating without data loss. It also helps in planning the right infrastructure size based on the criticality of workloads. Some virtual machines may require higher fault tolerance than others, which is why different FTT policies can be applied across the cluster.
For instance, a VDI environment might only need FTT = 1, while mission-critical applications could require FTT = 2 or even FTT = 3. Each FTT setting affects the number of data copies stored, which in turn influences storage usage. Earlier versions of vSAN relied heavily on mirroring, but newer releases support erasure coding, allowing more efficient use of storage resources.
To create or modify a virtual machine storage policy, log in to the vSphere Web Client and navigate to Home > Virtual Machine Storage Policy > New. From there, you can choose between RAID-1 (mirror) and RAID-5/6 (erasure coding). Note that RAID-5 and RAID-6 features are only available with a vSAN Enterprise license.
By using these policies, administrators can tailor the resilience of each workload to match its importance. This flexibility makes vSAN an ideal solution for environments with varying requirements. Whether you're managing a small cluster or a large-scale enterprise setup, understanding FTT and how it integrates with VM storage policies is essential for ensuring high availability and performance.
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