NOTE: You can mix x4 and x8 DRAM based DIMMs to support RAS features. However, all guidelinesfor specific RAS features must be followed. x4 DRAM based DIMMs retain Single Device DataCorrection (SDDC) in memory optimized (independent channel) mode. x8 DRAM based DIMMsrequire Advanced ECC mode to gain SDDC.The following sections provide additional slot population guidelines for each mode:Advanced ECC (lockstep)Advanced ECC mode extends SDDC from x4 DRAM based DIMMs to both x4 and x8 DRAMs. Thisprotects against single DRAM chip failures during normal operation.The installation guidelines for memory modules are as follows:• Memory modules must be identical in size, speed, and technology.• DIMMs installed in memory sockets with white release levers must be identical and the same ruleapplies for sockets with black release levers. This ensures that identical DIMMs are installed inmatched pair —for example, A1 with A2, A3 with A4, A5 with A6, and so on.NOTE: Advanced ECC with mirroring is not supported.Memory optimized (independent channel) modeThis mode supports SDDC only for memory modules that use x4 device width, and this mode does notimpose any specific slot population requirements.Memory sparingNOTE: To use memory sparing, this feature must be enabled in System Setup.In this mode, one rank per channel is reserved as a spare. If persistent correctable errors are detected ona 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 onerank per channel. For example, in a dual-processor configuration with sixteen 4 GB dual-rank memorymodules, the available system memory is: 3/4 (ranks/channel) × 16 (memory modules) × 4 GB = 48 GB,and not 16 (memory modules) × 4 GB = 64 GB.NOTE: Memory sparing does not offer protection against a multi-bit uncorrectable error.NOTE: Both Advanced ECC/Lockstep and Optimizer modes support memory sparing.Memory mirroringMemory mirroring offers the strongest memory module reliability mode compared to all other modes,providing improved uncorrectable multi-bit failure protection. In a mirrored configuration, the totalavailable system memory is one half of the total installed physical memory. Half of the installed memoryis used to mirror the active memory modules. In the event of an uncorrectable error, the system switchesover to the mirrored copy. This ensures SDDC and multi-bit protection.The installation guidelines for memory modules are as follows:• Memory modules must be identical in size, speed, and technology.• Memory modules installed in memory module sockets with white release levers must be identical andthe same rule applies for sockets with black and green release tabs. This ensures that identicalmemory modules are installed in matched pairs—for example, A1 with A2, A3 with A4, A5 with A6, andso on.61
