One of the new features in vSphere 5.5 is Flash Read Cache, and a post on blogs.vmware.com links to a Technical Paper outlining Flash Read Cache features, requirements, and usage cases. Boiled down to the basics, the Flash Read Cache feature takes host attached SSD drives or PCIe Flash cards and places them in a Flash Pool, which vSphere then manages and allocates as needed. The requirements for this to work are vSphere 5.5, virtual machines using hardware version 10 (VMX-10), and “at least one enterprise-level SSD or PCIe Flash card. Check the VMware Hardware Compatibility List (HCL) for an up-to-date list of supported Flash-based devices.”
Moving on to the VMware HCL page, setting the search to VMware Flash Read Cache, and not putting any restrictions on vendors or devices, yields 7 compatible SSD drives (and no listed PCIe Flash cards):
|Partner Name||Model||Interface Type|
|DELL||400GB Solid State Drive SAS Value SLC 6Gbps 2.5in||SAS|
|Samsung||SM1625 SAS SSD1||SAS|
It’s not an extensive list for HCL approved drives, but given that vSphere 5.5, and the Flash Read Cache feature itself, are pretty new, I would expect additions to the list over time. Regardless – even if desktop and laptop SSD prices are coming down a bit, enterprise SSD is still an expensive proposition.
So – if you upgrade to vSphere 5.5, and make it past the requirements and HCL, would Flash Read Cache be worthwhile for your VMs? The Technical Paper outlines two use cases for Flash Read Cache:
1) Enterprise level servers with high Read workloads – for example, data warehousing or web proxy servers.
2) Using Flash Read Cache as the swap file for memory overprovisioned VM hosts. One of the conclusions of a study on the effects of memory overprovisioning was that using SSD drives (or, in this case, a Flash Pool), somewhat mitigated the problem of storage latency issues in a hard memory state.
If your VMware environment is experiencing high disk read latency, Flash Read Cache may be a way to improve performance for your VMs. Keep in mind that this won’t address every disk latency issue, and there are both hardware and software version requirements to meet before you can test this out. However, I am looking forward to reading about both lab and real-world results for Flash Read Cache to see how much performance improves .