Cache Tier, Capacity Tier, FTT, vsan, vsphere

How much i get : Capacity Tier Calculation in Different FTT Scenarios

Hello All,

Someone asked me about the way to calculate the raw Capacity(HDD or SDD placed for Capacity tier), i elaborated him the way to organize and decided to share it with everyone.

For redundancy and high avialbility vsa offer Number of Failures to Tolerate (#FTT) which we can set to 0, 1, 2, 3 according to the need and capacity available. The default is #FTT=1 which means using distributed software RAID there will be 2 (#FTT+1) copies of the data on two different hosts in the cluster. So if the VM is 100GB then it takes 200GB of VSAN capacity to satisfy the protection. This would be analogous to RAID 1 on a storage array. Below is the chart for the different capacity scenario with different FTT level.

FTT Copies (FTT+1) RAW-to-usable Capacity %
0 1 100%
1 2 50%
2 3 33%
3 4 25%


Note : rather than writing to a disk then to another disk in the same host we write to another disk on another host in the cluster. With #FTT=1, VSAN can tolerate a single SSD failure, a single HDD failure, or a single host failure and maintain access to data. Valid settings for #FTT are 0, 1, 2, 3. If set to 3 then there will be 4 copies of the VM data thus RAW to usable would be 25%.

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