Editing Hyper-converged Infrastructure (section)

== Flash-to Spindle Ration and the Working Set Size ==
Assuming you are not using an all-flash [[HCI]] solution, to ensure your lab has the optimum performance, you will want the active data in your VMs to stay within the flash tier most of the time. If you cannot afford enough flash to do this, your lab will always be limited to the performance of its slowest component (i.e. your spindles).

As per [http://cormachogan.com/2014/11/14/tips-for-a-successful-virtual-san-proof-of-concept-poc/ this article from VMware's Chief Storage Guru], [https://twitter.com/CormacJHogan Cormac Hogan] when designing for [[VMware VSAN]]:

''The design goal should be for your application’s working set to be mostly in cache. VMs running on VSAN will send all their writes to cache, and will always try to read from cache. A read cache miss will result in data being fetched from the magnetic disk layer, thus increasing latency. This is an important design goal, and VMware recommends flash be sized at 10% of consumed VMDK size (excluding any failures to tolerate settings/mirror copies), but you may want to use more depending on your workloads. This value represents a typical working set.''

''Keep in mind, that as you scale up by adding new larger magnetic disks for capacity, and add more virtual machine workloads, you will have to increase your flash capacity too, so plan ahead. It might be good to start with a larger flash size to begin with if you do plan on scaling your workloads on VSAN over time.''