In combination with the storage controller, the SSD for the cache tier is the most important choice you can make in terms of the long-term reliability and performance of your VSAN deployment. The cache-tier SSD is used for caching reads and writes in VSAN hybrid configurations, and for caching writes only in all-flash configurations. SSDs are graded in many ways, and these grades for performance, write resiliency, and fabrication technology will all affect your selection. The VSAN Compatibility Guide gives you an overview of all of these factors to help you make the best choice.
Note
For the SSD:
The write-performance class is on a scale of A-F, with the F class being the fastest (class A is technically deprecated).
Write-resiliency is on a scale of A-D, with D being the most resilient.
Fabrication technology is either multi-layer cell (MLC), or single-layer cell (SLC). SLC is usually more performant and resilient, at the cost of lower capacities and higher price.
SSDs come in either SATA, SAS, or PCIe connections. PCIe SSD cards are usually at the top end in terms of performance. Given that SAS and PCIe interfaces permit significantly higher device command queues, it is a good idea to use SAS or PCIe type SSDs, especially if you are using a pass-through/JBOD controller that directly leverages device-level command queues.
The Compatibility Guide for SSD is navigated in the same way as for the I/O controller.
In the leftmost pane, select SSD.
In the next pane, select the most recent vSphere 5.5 or 6.0 update release.
In the next pane, select a brand name if desired.
Specify whether are searching for All Flash or Hybrid cache tier.
In the various other fields and drop-down menus, you can select the interface type, manufacturer, performance class, capacity, and form-factor.
After making your selections, choose Update and View Results to get a list of hardware that matches your specifications:
Determining the size of your capacity-tier (magnetic or SSD) storage will directly affect the sizing decision for your cache-tier SSD. In general, the SSD should be 10 percent of the size of the magnetic disks in the underlying disk group, for example, if your disk group will consist of four 1.5 TB capacity-tier disks, you will have 6 TB of bulk storage. To accommodate this, the SSD should be about 600 GB. Alternatively, you may choose to opt for two disk groups consisting of three 1TB capacity disks each. In this case, each disk group should have a cache-tier SSD of 300GB. The ratio of cache-tier SSD to capacity-tier disks or SSDs should be approximately 1:10.
For additional guidance regarding how to appropriately size the VSAN capacity and cache tiers, please see the section Chapter 1 – VSAN Capacity Planning of Appendix A, Chapter-specific Expansions.