I had a question on my old VMFS LUN Sizing article I did back in 2009… The question was how valid the used formula and values still were in today’s environment especially considering VMFS-5 is around the corner. It is a very valid question so I decided to take my previous article and rewrite it. Now one thing to keep in mind though is that I tried to make it usable for generic consumption and you will still need to figure out things yourself as I simply don’t have all info needed to make it cookie-cutter, but I guess this is as close as it can get.
MinSize = 1.2GB
MaxVMs = 40
SlackSpace = 20%
AvgSizeVMDK = 30GB
AvgDisksVMs = 2
AvgMemSize = 3GB
Before I will drop the formula I want to explain the MaxVMs parameter. You will need to figure out how many IOps your LUN can handle first, for a hint check this article. But besides IOps you will also beed to take burst room into account and of course the RTO defined for this environment:
((IOpsPerLUN – 20%) / AVGIOpsPerVM) ≤ (MaxVMsWithinRTO)
Keep in mind that the article I pointed out just a second ago is geared towards worst case numbers, so no cache or other benefits. Secondly I subtracted 20% which is room for bursting. Now this is by no means a best practice and this number will need to be tweaked based on the size of your LUN and the total amount of IOps you LUN can handle. For instance when you are using 8 SATA spindles that 20% might only be 80 IOps, depending on the raid level used, in the case of SAS it could be 280 IOps with just 8 spindles and that is a huge difference. Anyway I leave that up to you to decide but I used 20% headroom for both disk space (for snapshots and the memory overhead swap files) and performance, just to keep it simple. The second part of this one is MaxVMsWithinRTO. In short make sure that you can recover the number of VMs on the datastore within the defined recovery time objective (RTO). You don’t want to find yourself in a situation where the RTO is 4hrs but the total amount of time for the restore is 24 hours.
Formula, aaahhh yes here we go. Now note that I did not take traditional constraints around “SCSI Reservations Conflicts” into account as with VMFS -5 and VAAI SCSI Locking Offload these are lifted. If you have an array which doesn’t support the ATS primitive make sure you take this into account as well. Although the SCSI locking mechanism has been improved over the last years it could still limit you when you have a lot of power-on events, vMotion events etc.
(((MaxVMs * AvgDisksVMs) * AvgSizeVMDK) + ( MaxVMs * AvgMemSize)) + SlackSpace ≥ MinSize
Lets use the numbers defined in the parameters above and do the math:
(((40 * 2) * 30GB) + (40 * 3GB)) + 20% = (2400GB + 120GB) * 1.2 = 3024 GB
I hope this helps making your storage design decisions. One thing to keep in mind of course is that most storage arrays have optimal configurations for LUN sizes in terms of performance. Depending on your IOps requirements you might want to make sure that these align.