When we start a new project, I got the same question from top managers, what is the available capacity in our vSAN cluster? in fact, the hidden question was: what is the raw vs used capacity of the vSAN cluster, and how to plan our project.
I used to find some online documents and calculators to find it out. Today, I am trying to make it simple and up to the point in one blog.
To get to know vSAN, you need to know one important term: FTT (Failures To Tolerate) number.
vSAN is not like any kind of storage using hardware RAID, vSAN protection policy setting is “Number of Failures to Tolerate (#FTT) and can be set to 0, 1, 2, 3. The default is FTT number is 1 (similar to RAID 1 or mirroring) which means using distributed software RAID there will be 2 (#FTT+1) copies of the data on two different hosts in the cluster.
for example, if I have a virtual machine with 500GB, it will take 1TB from vSAN cluster to satisfy its level of protection.
So, instead of using RAID technology to write on a disk and then in the other disk in the host, vSAN writes on another disk in another host in the cluster. Then, vSAN can tolerate a single SSD failure, a single HDD failure, or a single host failure and maintain access to data.
As per the last example, it can be done on any valid FTT numbers (0,1,2,3). Note that, vSAN keeps few megabytes for overhead, but it can be nothing in your sizing calculation.
Below, FTT number and equivalent number of copies and raw vs usable capacity:
| Number of Failures to Tolerate | Number of copies (FTT+1) | % Raw-to-Usable Capacity |
| 0 | 1 | 100% |
| 1 | 2 | 50% |
| 2 | 3 | 33% |
| 3 | 4 | 25% |
Reading more about planning vSAN capacity from here, I also used this simple online calculator to help me as well as VMware official sizing tool from here (login credentials needed).
Former Nuclear Engineer | University Lecturer | Technology Advisor | Digital Transformation evangelist | FinTech | Blockchain | Podcaster | vExpert ⭐️⭐️⭐️⭐️ | VeeamVanguard ⭐️⭐️ | Nutanix SME | MBA | AWS ABW Grant’23
