Dell PowerVault ME5 Series Administrator's Guide

About SSD read cache

Unlike tiering, where a single copy of specific blocks of data resides in either spinning disks or SSDs, the Read Flash Cache (RFC) feature uses one SSD read-cache disk group per pool as a read cache for frequently accessed data only. Each read-cache disk group consists of one or two SSDs with a maximum usable capacity of 4TB. A separate copy of the data is also kept in spinning disks. Read-cache content is lost when a controller restart or failover occurs. Taken together, these attributes have several advantages:

• The performance cost of moving data to read-cache is lower than a full migration of data from a lower tier to a higher tier. ● Read-cache does not need to be fault tolerant, potentially lowering system cost.
• Controller read cache is effectively extended by two orders of magnitude, or more. When a read-cache group consists of one SSD, it automatically uses NRAID. When a read-cache group consists of two SSDs, it automatically uses RAID 0.

For more information on SSDs, see About SSDs