Internally someone asked this question, and at the Italian VMUG I had someone asking me the same question… What if I want to scale out, or scale-up, and need to add differently sized disks to my existing VSAN environment? Will that be an as expensive exercise as with (some) traditional RAID systems?
Some of you have introduced new disks to RAID sets in the pasts may have seen this: you add a 2TB disk to a RAID config that only has 1TB disks and you waste 1TB as the RAID set only includes the capacity of the other disks. VSAN is not like this fortunately!
With VSAN you can scale-up and scale-out dynamically. VSAN does not, to a certain extend, care about the disk capacity. For VSAN the disk is just destination to store objects, and there is no filesystem or lower level formatting going on to stripe blocks across the disks, sure it uses a filesystem… but this is “local” to the disk, and not across disks. So whether it is a 1TB disk you add to an environment with all 1TB disks, or you add a 2TB disk, it will not matter to VSAN. Same applies to replacing disks by the way, if you need to replace a 1TB disk because it has gone bad and would like to use a 2TB disk instead… go ahead! Each disk will have its own filesystem, and the full capacity can be used by VSAN!
The question then arises, will it make a difference if I use RAID-0 or Passthrough at the disk controller level? Again, it does not. Keep in mind that when you do RAID-0 configurations for VSAN that each disk is in its own RAID-0 configuration. Meaning that if you have 3 disks, you will have 3 x RAID-0 set each containing 1 disk. Of course, there is a small implication here when you replace disks as you will need to remove the old RAID-0 set with that disk and create a new RAID-0 set with the new disk, but that is fairly straight forward.
One thing to keep in mind though, from an architectural / operational perspective… if you swap out a 1TB disk for a 2TB disk then you will need to ask yourself will this impact the experience for my customers. Will the performance be different? Because 100 IOps coming from the same disk for 1TB is different then 100IOps coming from the same disk for 2TB, as you will (simply said) be sharing the 100IOps with more VMs (capacity). In short: 100 IOps for a 1000GB disk = 0,1 IOps per GB BUT 100 IOps for a 2000GB disk = 0,05 IOps per GB, you can see the potential impact right… You have more capacity per disk, but with the same number of IOps being provided by that disk. Hopefully though the majority of IO (all writes will for sure, and most reads) will be handles by flash so the impact should be relatively low. Still, something to consider.
