As of time of writing this article we have lots of information on the datastore sizing especially for VMFS. This hasn’t changed too much with new versions of VMFS and if you have taken into account a good buffer of slack space (Usually vary between 20%-30% per VMFS) you should be good with your old calculations.
Let’s revisit some of the parameters you use when you do calculate your VMFS Datastore size.
Total No Of VMs per Lun : The general recommendation for this in server workloads is to have around 40-45 vms , however this is not a final number and this is affected by few of the things mentioned below
- SCSI Reservations: SCSI Reservation conflicts can happen, if you have lots of parallel power on operations, vMotion, sVMotion etc. However with VAAI on the new arrays this is much better handled.
- Recovery considerations: you also need to plan keeping in mind your recovery plans (RTO) , you don’t want to end up in a situation where you are not able to restore VMs within the specified RTO.
Size of the VM Disk: This is driven by what type of function the VM is going to do , however having a standard template does help in these situations. Make sure that you don’t over provision the disk space.
VM Memory Size: We very well know that there is an equal amount of .vswp (swap) file created on the VMFS datastore as the assigned memory of the VM. However if you do have reservations the .vswp file size will be calculated as
.vswp size = (Total assigned VM Memory – Total Reserved Memory)
For Example .vswp size= (4GB-2GB) for a VM with 4 GB assigned memory and 2 GB Memory reservation will be 2 GB.
Other parameters that you look at from the VMFS datastore size perspective is the use of snapshots and some space reserved for bursts , this usually is recommended to be around the 30% mark combining both.
There are few important considerations that you need to take from storage characteristic perspective such as IOPS, Read write ratios, RAID Type, Disk RPM speeds.
To simplify these considerations I have created a VMware VMFS Datastore Sizing Calculator which will take all these values into consideration and give you a figure which you can use in your design and also to speak with your storage architect. One of the important considerations to take when using this calculator is that the calculator is based on some industry standards and your Array vendor may have some variations with the data, in that case please discuss this with the storage vendor and arrive at a proper sizing.
IOPS Considerations for Calculator:
RAID Write Penalty Considerations for Calculator:
Please do take your time and provide feedback for further improvement of the Calculator.
