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Memory module installation order

Memory modules must be installed in a specific order based on the memory configuration that you implement and the number of processors and memory modules installed in the compute node.

Note

List of supported memory module is different for 1st generation (Skylake) and 2nd generation (Cascade Lake) Intel Xeon processors. Make sure to install compatible memory modules to avoid system error. For a list of supported DIMMs, see: Lenovo ServerProven website.

The following memory configurations and population sequences are supported for the ThinkSystem SN850:

Memory installation requirements:

  • A label on each DIMM identifies the DIMM type. This information is in the format xxxxx nRxxx PC4-xxxxx-xx-xx-xxx. Where n indicates if the DIMM is single-rank (n=1) or dual-rank (n=2).

  • At least one DIMM is required or each processor. Install at least six DIMMs per processor for good performance.

  • The following table includes all the feasible combinations of different types of DIMMs:

    Table 1. DIMM compatibility
    DIMM TypesRDIMMLRDIMM3DS RDIMM
    RDIMMVXX
    LRDIMMXVX
    3DS RDIMMXXV

  • When you replace a DIMM, the compute node provides automatic DIMM enablement capability without requiring you to use the Setup utility to enable the new DIMM manually.

Attention

  • Mixing x4 and x8 DIMMs in the same channel is allowed.

  • Install DIMMs of the same speed for optimal performance. Otherwise, the BIOS will find and run the lowest speed among all channels.

  • Always populate DIMMs with the maximum number of ranks in the farthest DIMM slot, followed by the nearest DIMM slot.

  • Independent memory mode
    In independent memory mode, memory channels can be populated with DIMMs in any order and you can populate all channels for each processor in any order with no matching requirements. Independent memory mode provides the highest level of memory performance, but lacks failover protection. The DIMM installation order for independent memory mode varies based on the number of processors and memory modules installed in the compute node.
  • Memory mirroring
    Memory-mirroring mode provides full memory redundancy while reducing the total system memory capacity in half. Memory channels are grouped in pairs with each channel receiving the same data. If a failure occurs, the memory controller switches from the DIMMs on the primary channel to the DIMMs on the backup channel. The DIMM installation order for memory mirroring varies based on the number of processors and DIMMs installed in the compute node.
  • Memory sparing
    In memory-sparing mode, one memory rank serves as a spare for other ranks on the same channel in case they fail. The spare rank is held in reserve and not used as active memory until a failure is indicated, with reserved capacity subtracted from the total available memory in the system. The DIMM installation order for memory sparing varies based on the number of processors and memory modules installed in the compute node.
  • DC Persistent Memory Module — Memory Mode
    In this mode, DCPMMs act as volatile system memory, while DRAM DIMMs act as cache. Only DCPMM capacity is displayed as system memory in this mode.