vsphere-esxi-vcenter-server-55-storage-guide

About VMware vSphere Flash Read21CacheFlash Read Cache lets you accelerate virtual machine performance through the use of host resident flashdevices as a cache.You can reserve a Flash Read Cache for any individual virtual disk. The Flash Read Cache is created onlywhen a virtual machine is powered on, and it is discarded when a virtual machine is suspended or poweredoff. When you migrate a virtual machine you have the option to migrate the cache. By default the cache ismigrated if the virtual flash module on the source and destination hosts are compatible. If you do notmigrate the cache, the cache is rewarmed on the destination host. You can change the size of the cache whilea virtual machine is powered on. In this instance, the existing cache is discarded and a new write-throughcache is created, which results in a cache warm up period. The advantage of creating a new cache is that thecache 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 asa SAN or NAS. All data that is read from or written to the backing storage is unconditionally stored in thecache.NOTE Not all workloads benefit with a Flash Read Cache. The performance boost depends on yourworkload pattern and working set size. Read-intensive workloads with working sets that fit into the cachecan benefit from a Flash Read Cache configuration. By configuring Flash Read Cache for your read-intensiveworkloads additional I/O resources become available on your shared storage, which can result in aperformance increase for other workloads even though they are not configured to use Flash Read Cache.This chapter includes the following topics:• “DRS Support for Flash Read Cache,” on page 206• “vSphere High Availability Support for Flash Read Cache,” on page 206• “Using Virtual Flash Resource to Configure a Swap Cache,” on page 206• “Virtual Flash Volume Limits,” on page 206• “Set Up Virtual Flash Resource,” on page 206• “Set Up Multiple Virtual Flash Resources,” on page 207• “Configure Host Swap Cache with Virtual Flash Resource,” on page 207• “Configure Flash Read Cache for a Virtual Machine,” on page 208• “Remove Virtual Flash Resource,” on page 208• “Migrating Virtual Machines with Flash Read Cache,” on page 209• “Local SSDs Are Unavailable for Use with Virtual Flash,” on page 209• “Using esxcli Commands with Virtual Flash,” on page 210VMware, Inc. 205