RAID-1 (Mirroring)

[Vefasto]

Hi everybody,

i have a farm with three hosts on the same site configured with policy RAID-1 Mirroring.

What happens if one host fails and one disk fail  on the remaining active hosts?

Thank You

[Alexander Schneider]

It depends :)

Assuming you are using VSAN OSA?

With three hosts, you have 2 copies of your VM objects and a witness object, distributed across those three hosts.
If you lose one host, you still have either 2 copies of your data OR 1 copy + witness object, your workloads keep on running, but you cannot create any new objects/VMs in that cluster.

Now, to the "it depends" part:

• In case of only one capacity drive in your VSAN Disk Groups, and this one drive fails, all VMs will be affected and stop I/O.
• If you have more than one capacity disk per host in your VSAN Disk Groups (and not using Deduplication), it depends on which additional disk fails, because there might be a component of VM 1 on it, but not VM 2. Then VM 1 will stop working (I/O will be halted), while VM 2 still has enough components (more than 50%).
• And in case of a cache drive failure (assuming you use VSAN OSA), it will affect the whole disk group anyways, so it doesn't matter if you have multiple capacity drives.

[Vefasto]

Thank You Alexander, start to be clear.

I checked the vsan cusotmer configuration and i confirm that use OSA(is an old implementation) with  two groups composed each  by three capacity Tier drives and one Cache Tier.

Regards