VxRail: Storage Usable capacity

So What would be a good estimate to provide customers with when asked the question "How much usable capacity can I expect from a VxRail Appliance?"

Also, I remember someone mentioning that the FTT setting on a hybrid appliance is set to 1 by default and cannot be changed. Is this true or can the customer specify the FTT setting per VM and choose between FTT=0,1,2 and 3?

Thanks.

I'll answer the easy question first. FTT value can be set per VMDK in VSAN on all VxRail models (the all-flash models also offer EC-1 and EC-2 as choices).

The quick, unqualified answer to how much usable capacity can I expect with VxRail is "up to nearly 50% of raw capacity" for hybrid systems and "up to nearly 75% of raw capacity" for all-flash systems.

In order to provide a more accurate answer, you need to understand your customer's workload:

What FTT or EC value?

How much of the allocated storage actually be utilized on average?

How many IOPS?

FFT and EC value obviously have a huge effect on usable capacity.

The latter two questions pertain to the amount of slack space (headroom) that's required. VMware has a best practice for VSAN to leave 30% of the formatted capacity (before FTT) unused as slack space. That slack space is intended to prevent any capacity drive from exceeding 80% utilization. When that happens, VSAN automatically rebalances the datastore, and that can consume 15-20% of the available IOPS. You cannot turn off this automatic rebalancing, but you may be able to reduce the chance that it happens by regularly manually invoking a rebalance in off-hours.

In my opinion, 30% slack space is overly conservative, as it's only really an issue if the rebalance occurs while the VxRail cluster is running over 80% of its performance capacity. If VxRail is running at less than 80% of its performance capacity, then 15-20% fewer IOPS being available for the workload shouldn't affect the performance of a workload.

Furthermore, since VSAN by default thin-provisions storage allocated to VMs, and by default doesn't allow storage over-subscription, it's unlikely an automatic rebalance will occur if the VMs actually utilize on average less than ~75% of their allocated storage. I believe it's fairly common for storage utilization to be significantly lower than storage allocation, especially for VDI.

I hope this helps.

Richard

This is incredibly useful! Thanks a lot Richard!