Confusion on RAID 5 VSAN FTT

[ManivelR]

I have a current setup with 4 nodes with FTT=1. I mean RAID 5.

As per my current scenario,one node can go down at the time and remaining 3 nodes will be operational.During one node failure, existing operations will work but the new operations(new vm creation,snapshot creations,cloning etc..) will not work as we have only available 3 nodes.

Lets assume:-

5 nodes setup with FTT=1. I mean RAID 5.

During this setup,2 nodes can go down right at the same time ?

6 nodes setup with FTT=1. I mean RAID 5.

During this setup,2 nodes can go down right at the same time ?

Note:- RAID5 with FTT of 1 needs a minimum of 4 hosts, if one or 2 nodes fail, I will still be able to create VM since the minimum requirement for RAID5 with FTT of 1 is to have 4 nodes.

Whenever I use 5 nodes or 6 nodes,at the same time 2 nodes can go down ?

3 nodes FTT is applicable to only RAID 1 not RAID 5 ?

Thanks,

Manivel R

[a_p_]

Unless I'm missing something, FTT (Failures to tolerate) basically pretty straight forward:

FTT=1 -> one failure can be tolerated

FTT=2 -> two failures can be tolerated

André

[KabirAli82]

Here is a easy to read table explaining FTM:

For more reading:

RAID-5/RAID-6 Erasure Coding | vSAN 6.7 U 3 Proof of Concept Guide | VMware

Host Requirements | vSAN Space Efficiency Technologies | VMware

[depping · Accepted solution]

You need to look at it from an OBJECT or VM point of view. If you have 5 hosts and an object with RAID-5 it could be that it is a standard RAID-5 deployment, meaning 3+1 (3 data + 1 parity) and that they are spread across 4 hosts as shown in the diagram.

if the above is the case and host 5 and host 4 go down, then there's no impact to the data of this particular RAID-5 configuration. This is because host 5 is not containing any components of the RAID-5 set and as such the Object is only experiencing 1 failure, while your cluster is experiencing 2 failures.

So whenever you use 5 or 6 nodes it could indeed be that 2 hosts go down and VMs are not impacted, it could also however be that VMs are impacted. As in the above scenario if  2 hosts would go down then your FTT=1 configuration with RAID-5 is experiencing 2 failures.

[ManivelR]

Thanks so much Depping and everyone.