Dell PowerEdge M620 Owner's Manual page 208

Non-Optimal Memory Configurations
System performance can be affected if your memory configuration does not
conform to the preceding installation guidelines. Your system may issue an
error message during start-up stating that your memory configuration is non-
optimal.
Memory Sparing Support – PowerEdge M605
Memory sparing is supported in single- or dual-processor systems that have one
of the fully populated memory configurations shown in Table 3-13. The
memory sparing feature must be enabled in the Memory Information screen
of the System Setup program. See "Memory Settings Screen" on page 140. To
use memory sparing, you must disable node interleaving.
Memory sparing allocates only the first rank of memory of a DIMM. For a
single-rank DIMM, the entire capacity of the DIMM must be allocated for
sparing along with the adjacent single-rank DIMM to spare a memory
channel. For dual-rank DIMMs, two DIMMs are also required for sparing, but
as only the first rank of each DIMM is allocated, only half of a dual-rank
DIMM's capacity is allocated for sparing. The second ranks on both DIMMs
are available memory. Table 3-13 shows how memory sparing splits the
available and spared memory in each memory module configuration.
208
Installing Blade Components