116 Installing System ComponentsMemory Sparing SupportMemory sparing is supported in systems that have one of the fully populatedmemory configurations shown in Table 3-1. The memory sparing featuremust be enabled in the Memory Information screen of the System Setupprogram. See "Using the System Setup Program" on page 47.NOTE: The Memory sparing and node interleaving features cannot be used at thesame time. To use memory sparing, you must disable the Node Interleaving optionin the System Setup program.Memory sparing is applied independently to the two groups of DIMMs onopposite sides of the processor sockets. To support memory sparing, allDIMM sockets within a DIMM group must be populated.When enabled, memory sparing allocates and reserves ranks of memory fromthe installed DIMMs to act as spare memory in the event of a memorychannel failure. A memory channel uses paired DIMMs; for memory sparingto work, DIMMs must be paired as spares also.Memory sparing allocates only the first rank of memory of a DIMM. For asingle-rank DIMM, the entire capacity of the DIMM must be allocated forsparing along with the adjacent single-rank DIMM to spare a memorychannel. For dual-rank DIMMs, two DIMMs are also required for sparing, butas only the first rank of each DIMM is allocated, only half of a dual-rankDIMM’s capacity is allocated for sparing. The second ranks on both DIMMsare available memory.Memory sparing calculates the appropriate DIMMs to spare by searching theDIMM 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 tospare one of the available channels of memory. If the DIMMs in these socketsare sufficient to spare a channel of the available memory, the first rank of eachof those two DIMMs are spared. If the amount of memory is not large enoughto spare an available memory channel, the system spares the DIMM ranks inthe lower-numbered sockets. Table 3-2 shows how memory sparing works invarious memory configurations.
