Dell PowerEdge R520 Owner's Manual

In this mode, one rank per channel is reserved as a spare. If persistent correctable errors are detected on a rank, the data from this rank is copied to the spare rank and the failed rank is disabled.

With memory sparing enabled, the system memory available to the operating system is reduced by one rank per channel. For example, in a system with three 8 GB dual-rank DIMMs, the available system memory is: 1/2 (ranks/channel) × 3 (DIMMs) × 8 GB = 12 GB, and not 3 (DIMMs) × 8 GB = 24 GB.