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116 Installing System Components
Memory Sparing Support
Memory sparing is supported in systems that have one of the fully populated
memory configurations shown in Table 3-1. The memory sparing feature
must be enabled in the Memory Information screen of the System Setup
program. See "Using the System Setup Program" on page 47.
NOTE: The Memory sparing and node interleaving features cannot be used at the
same time. To use memory sparing, you must disable the Node Interleaving option
in the System Setup program.
Memory sparing is applied independently to the two groups of DIMMs on
opposite sides of the processor sockets. To support memory sparing, all
DIMM sockets within a DIMM group must be populated.
When enabled, memory sparing allocates and reserves ranks of memory from
the installed DIMMs to act as spare memory in the event of a memory
channel failure. A memory channel uses paired DIMMs; for memory sparing
to work, DIMMs must be paired as spares also.
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.
Memory sparing calculates the appropriate DIMMs to spare by searching the
DIMM sockets, starting with the higher-numbered socket pair (specifically,
sockets 3 and 4 or sockets 7 and 8), for an amount of memory large enough to
spare one of the available channels of memory. If the DIMMs in these sockets
are sufficient to spare a channel of the available memory, the first rank of each
of those two DIMMs are spared. If the amount of memory is not large enough
to spare an available memory channel, the system spares the DIMM ranks in
the lower-numbered sockets. Table 3-2 shows how memory sparing works in
various memory configurations.