If there is a need for >8 hosts, then I'd go for 8 hosts per cluster.
Reasons why 8, not 4 or 12 or 16 or 32 hosts:
- Best practice for cluster is to give is same hardware spec with same CPU frequency.
- Eliminates risk of incompatibility
- Complies with Fault Tolerant best practices
- So more than 8 means it’s more difficult/costly to keep them all the same. You need to buy 8 hosts a time, which can be expensive.
- Too few hosts result in overhead (the “spare” host)
- Too many hosts are harder to manage (patch, performance troubleshooting, too many VMs per cluster, HW upgrade)
- Some cluster changes in the Advanced Attributes requires cluster to be disable and enable.
- It is harder/longer to do this when there are many hosts
- 8 hosts give DRS sufficient space to “maneuver”
- #VM per host decrease by 4x in >8 host.
- 160 VMs per host if <= 8 hosts in cluster
- 40 VMs per host if > 8 hosts in cluster
- We should avoid being near the limit.
- 40 VM/host is easily reached in Lab Manager or View environment.
- Availability: Able to withstand 1-2 host failures
- A balance between too small (4 hosts) and too large (>12 hosts)
- Allow us to isolate 1 host for VM-troubleshooting purpose
- Upgrading >8 servers at a time is expensive ($$) and complex
- Storage: 8 hosts/LUN à a safe value of 16 paths to a LUN
- Consistent with VMware View
- View 4 cluster is limited to 8 hosts if we use View Composer
- 8 is easy number to remember. And a lucky one. And we all know that production needs luck, not just experience :-)
That's for my first posting. I sincerely hope it's useful.