Choosing the Right Type and Capacity of SSD
SSD Performance – Enterprise SSDs come in three flavors SATA, SAS or PCIe. A rule of thumb is that SATA SSDs do 50K IOPS, SAS do 100K IOPS and PCIe do 150K IOPS, and they are proportionately priced as well. Also account for the fact that depending on the workload caching throughput for the ESXi server might degrade to 50% of raw SSD throughput. So worse case assume that you will get 25K IOPS with SATA, 50K IOPS with SAS, and 75K IOPS with PCIe Flash for EACH ESXi Server, so depending on the amount of acceleration required, choose the right type of SSD.
SSD Sizing – The size of the SSD installed in the ESXi server should be between 10-30% of the LUN/Disk capacity being used by all the VMs on the ESXi server. Some workloads are caching friendly (VDI for one) and others are not. Again a good rule of thumb would be to start with 10-20% of Disk/LUN capacity (utilized by the ESXi Server) as SSD capacity.