Memory rank sparingThis information provides an overview for memory rank sparing.The server supports memory rank sparing. Memory rank sparing reserves memorycapacity for failover in the event of a DIMM failure, and the reserved capacity issubtracted from the total available memory. Memory sparing provides lessredundancy than memory mirroring does. If a predetermined threshold ofcorrectable errors is reached, the contents of the failing DIMM are copied to thespare memory, and the failing DIMM or rank is disabled.To enable memory sparing through the Setup utility, select System Settings →Memory → Memory Mode. For more information and notes about installingDIMMs, see “Installing a memory module” on page 47. For more information andthe DIMM population sequence for memory rank sparing, see “Memory ranksparing in independent mode” on page 58 and “Memory rank sparing in lockstepmode” on page 62.Memory-sparing is supported in both independent memory mode and lockstepmemory mode.The DIMM installation order for memory rank sparing follows the independentmode (performance) or lockstep mode (RAS) installation order based on the modeof operation selected. For more information, see “Independent memory mode” onpage 55, and “Lockstep memory mode” on page 59.When you use the memory rank sparing feature, consider the followinginformation:v Memory rank sparing is not supported if memory mirroring is enabled.v The spare rank must have identical or larger memory capacity than all the otherranks on the same DDR3 channel.v When single-rank DIMMs (that is, 4 GB and 8 GB) are used, a minimum of tworank DIMMs must be installed per memory channel to support memory sparing.v When multi-rank DIMMs (that is, 16 GB, 32 GB, and 64 GB) are used, onemulti-rank DIMM can be installed per memory channel to support memorysparing.v The total memory available in the system is reduced by the amount of memoryallocated for the spare ranks.54 System x3850 X6 and x3950 X6 Types 3837 and 3839: Installation and Service Guide
