Home > Storage > PowerVault > Guides > Dell PowerVault ME5 Series: VMware vSphere Best Practices > Read flash cache
With tiering, a single copy of specific data blocks resides in either spinning disks or SSDs. However, the read flash cache feature uses one or two SSD disks per pool as a read cache for hot or frequently read pages only. Read cache does not add to the overall capacity of the pool to which it has been added, nor does it improve write performance. You can add read flash cache from the pool without any adverse effect on the volumes and their data in the pool, other than to impact the read-access performance. A separate copy of the data is always maintained on the HDDs. Taken together, these attributes have several advantages: