The storage policies applied to a stretched cluster would define the minimum number of hosts that are required per site.
For example, a site disaster tolerance set to “Dual site mirroring (stretched cluster)” and a Failure to tolerate set to “1 failure – RAID - 5 (Erasure coding)”, would result in three data blocks and one parity component, thus we would need at least four hosts per site. The final configuration will be 4+4+ 1, 4 hosts per site, and one witness host.
From a capacity standpoint, if you have a 100 GB VM and set the Site disaster tolerance to “Dual site mirroring (stretched cluster)” and a Failure to tolerate equal to one failure – RAID - 1 (Mirroring), means a RAID 1 is set in each site.
In this case, a 100 GB VM would require 200 GB in each location. So, 200% required local capacity, 400% for the total cluster. Using the following table, you can easily see the overhead. Note that RAID 5 and RAID 6 are only available when using all-flash.
Table 2. Stretched Cluster Host Configurations
No local protection | 0 | RAID 1 | 1 | 1+1+1 | 100% of VM | 200% of VM |
Local protection | 1 | RAID 1 | 3 | 3+3+1 | 200% of VM | 400% of VM |
Local RAID 5 | 1 | RAID 5 | 4 | 4+4+1 | 133% of VM | 266% of VM |
Local RAID 6 | 2 | RAID 6 | 6 | 6+6+1 | 150% of VM | 300% of VM |
