An all-flash vSAN cluster contains a sufficient number of capacity devices and cache devices to support different RAID levels.

FTT=3(RAID-1 Mirroring) - This policy requires at least 4 capacity devices, where each object is mirrored across three copies of the object. This can be satisfied by the 4 nodes all-flash vSAN cluster.

FTT=1 (RAID-1 Mirroring) - This policy requires at least 2 capacity devices, where each object is mirrored across one copy of the object. This can be satisfied by the 4 nodes all-flash vSAN cluster.

FTT=2 (RAID-6 Erasure Coding) - This policy requires at least 6 capacity devices, where each object is striped across two sets of parity and data. This cannot be satisfied by the 4 nodes all-flash vSAN cluster.

FTT=1 (RAID-5 Erasure Coding) - vSAN does not support RAID-5 Erasure coding.

When upgrading existing vSAN hosts with multiple SSD cache devices per host to NVMe devices, it is important to note that the disk group does not need to be removed before adding new cache. NVMe devices can be hot-plugged into the host, and vSAN will automatically detect and use the new devices as cache. It is important to check if the NVMe devices are supported by your version of vSAN and also check that the NVMe devices are on the VMware HCL.

This is explained in VMware vSAN documentation in the section "Upgrading vSAN Hardware" Reference: