Dell PowerVault ME5 Series Administrator's Guide

SSD read cache

Unlike tiering, where a single copy of specific blocks of data resides in either spinning disks or SSDs, the read cache feature uses one SSD read cache disk group per pool as a read cache for "hot" pages only. If a virtual pool contains a flash tier, then read-cache is not allowed.

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 contents are lost when a controller restart or failover occurs. This does not cause data loss or corruption, as the read cache only duplicates the content that exists in the fault-tolerant disk groups. 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 is not 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, see SSDs.