I had a meeting last week with a customer and a question came up around the minimum number of hosts a cluster requires in order to use. particular RAID configuration for vSAN. I created a table for the customer and a quick paragraph on how this works and figured I would share it here as well.
With vSAN ESA VMware introduced a new feature called “Adaptive RAID-5”. I described this feature in this blog post here. In short, depending on the size of the cluster a RAID-5 configuration will either be a 2+1 scheme or a 4+1 scheme. There’s no longer a 3+1 scheme with vSAN ESA. Of course, there’s still the ability to use RAID-1 and RAID-6 as well, the RAID-1 and RAID-6 schemes remained unchanged.
When it comes to vSAN ESA, below are the number of hosts required for a particular RAID scheme. Do note, that with RAID-5, of the size of the cluster changes (higher of lower) then the scheme may also change as described in the linked article above.
| Failures To Tolerate | Object Configuration | Minimum number of hosts | Capacity of VM size |
|---|---|---|---|
| No data redundancy | RAID-0 | 1 | 100% |
| 1 Failure (Mirroring) | RAID-1 | 3 | 200% |
| 1 Failure (Erasure Coding) | RAID-5, 2+1 | 3 | 150% |
| 1 Failure (Erasure Coding) | RAID-5, 4+1 | 6 | 125% |
| 2 Failures (Erasure Coding) | RAID-6, 4+2 | 6 | 150% |
| 2 Failures (Mirorring) | RAID-1 | 5 | 300% |
| 3 Failures (Mirorring) | RAID-1 | 7 | 400% |
Can you explain why it is not possible to keep the 3+1 scheme for a 4-ESXi vSAN ESA cluster ? It will permit a “cost” of 133% instead 150% with the actuel 2+1 scheme….
Theoretically that should indeed be possible, but the team decided to create a different implementation based on what customers asked for. I am happy to ask them to see if it is possible to also implement 3+1.
Hey Duncan. How did you go with this?
I think there will be a lot of people out there trying to best configure a 4-node ESA cluster.
To me neither RAID5 2+1 or RAID1 2+2 look ideal.
Surely the best would be either RAID5 3+1 or maybe even RAID6 2+2?
For the 5 node FTT2 RAID1, Can you also do 4+Witness?
A dedicated witness node can only be used when you have a stretched cluster configuration currently.
Hi Duncan,
Firstly thank you for the above information.
I just had a couple of questions.
How does this work over a stretched cluster?
What is the minimum number of hosts per site?
What would the scheme be if for example I had 4 hosts at each site.
Thank you in advance.