Flash Read Cache™ lets you accelerate virtual machine performance through the use of host resident flash devices as a cache.

The vSphere Storage documentation discusses vSphere Flash Read Cache in some detail.

You can reserve a Flash Read Cache for any individual virtual disk. The Flash Read Cache is created only when a virtual machine is powered on, and it is discarded when a virtual machine is suspended or powered off. When you migrate a virtual machine you have the option to migrate the cache. By default the cache is migrated if the virtual flash module on the source and destination hosts are compatible. If you do not migrate the cache, the cache is rewarmed on the destination host. You can change the size of the cache while a virtual machine is powered on. In this instance, the existing cache is discarded and a new write-through cache is created, which results in a cache warm up period. The advantage of creating a new cache is that the cache size can better match the application's active data.

Flash Read Cache supports write-through or read caching. Write-back or write caching are not supported. Data reads are satisfied from the cache, if present. Data writes are dispatched to the backing storage, such as a SAN or NAS. All data that is read from or written to the backing storage is unconditionally stored in the cache.

Note: Not all workloads benefit with a Flash Read Cache. The performance boost depends on your workload pattern and working set size. Read-intensive workloads with working sets that fit into the cache can benefit from a Flash Read Cache configuration. By configuring Flash Read Cache for your read-intensive workloads additional I/O resources become available on your shared storage, which can result in a performance increase for other workloads even though they are not configured to use Flash Read Cache.

You can manage vSphere Flash Read Cache from the vSphere Client. You can monitor Flash Read Cache by using commands in the esxcli storage vflash namespace. The following table lists available commands. See the ESXCLI Reference for a list of options to each command.

Table 1. Commands for Monitoring vSphere Flash Read Cache
Command Description
esxcli storage vflash cache get Gets individual vflash cache info.
esxcli storage vflash cache list Lists individual vflash caches.
esxcli storage vflash cache stats get Gets vflash cache statistics.
esxcli storage vflash cache stats reset Resets vflash cache statistics.
esxcli storage vflash device list Lists vflash SSD devices.
esxcli storage vflash module get Gets vflash module info.
esxcli storage vflash module list Lists vflash modules.
esxcli storage vflash module stats get Gets vflash module statistics.