To implement a RAID-6 SFTT, one would require 6 hosts at the primary site and 6 hosts at the secondary site, as well as a host to facilitate the witness appliance. This configuration is referred to as a 6+6+1 configuration. The SFTT Fault Tolerance Method (FTM) must be the same as both of the data sites; we cannot mix FTMs at this point in time. So with a RAID-6 FTM, a configuration might look something like this:
To implement a RAID-5 SFTT, one would require 4 hosts at both data sites, as well as a witness as a third site. This is a 4+4+1 configuration. RAID-5 is implemented as 3 data segments and 1 parity segment, and a configuration would look something like this:
This final SFTT is using RAID-1. This one seems to be slightly confusing as it is using a witness (or witnesses) locally at each data site for SFTT, as well as using a witness on the witness appliance to achieve PFTT. Now with RAID-1, we can tolerate 1, 2 or 3 failures. The rule of thumb is that to tolerate “n” failures, you need “2n+1” hosts. In this configuration, I have deployed SFTT to tolerate 1 failure, and set the FTM to RAID-1. Therefore this configuration requires a 3+3+1 setup. I’d expect a layout something similar to the following:
For FTM = RAID-1 to have an SFTT of 2, the configuration would need to be 5+5+1 (remember 2n+1 in this case is 5).
For FTM = RAID-1 to have an SFTT of 3, the configuration would need to be 7+7+1 (again, 2n+1 in this case is 7).
Although, using FTM of RAID-1 is going to consume a lot of space, so consider RAID-5 or RAID-6 for their space-saving techniques.