vSAN can be configured in one of two fault tolerance modes. RAID1 (Mirroring) delivers the best performance and RAID 5/6 (Erasure Coding) which is optimized for capacity. Erasure Coding does not utilize witness components, instead it uses data parity. There is an overhead to using Erasure Coding, since it has to perform the parity bit calculations. More information on Erasure Coding can be found in the vSAN 6.2 Design and Sizing Guide (see Section 10, References).