Memory mirroring in lockstep mode
Use this information for notes and details about using the memory mirroring feature in lockstep memory
mode.
Memory-mirroring replicates and stores data on DIMMs across all four memory channels for the compute
book and DDR3 or DDR4 channels simultaneously. If a failure occurs, the memory controller switches from
the DIMMs on the primary channels to the DIMMs on the backup channels.
To enable memory mirroring through the Setup utility, select System Settings → Memory → Memory Mode.
For more information, see “Using the Setup utility” on page 123. For more information and notes about
installing DIMMs, see “Installing a memory module” on page 46 and “Lockstep memory mode” on page 57.
Consider the following information when using the memory mirroring feature in lockstep memory mode:
• The compute book memory channel 0 mirrors memory channel 1, and memory channel 2 mirrors memory
channel 3. This mirroring provides redundancy in memory but reduces the total memory capacity in half.
• DIMMs must be installed across the memory channels for each compute book when using the memory
mirroring feature in lockstep mode.
• The DIMM population must be identical (size, organization, etc.) for memory channel 0 and memory
channel 1, and identical for memory channel 2 and memory channel 3.
• Memory mirroring reduces the maximum available memory by half of the installed memory. For example, if
the server has 64 GB of installed memory, only 32 GB of addressable memory is available when memory
mirroring is enabled.
• The following table lists the DIMM installation sequence for memory mirroring in lockstep memory mode:
Table 13. DIMM population sequence when memory-mirroring is enabled or disabled in lockstep mode
DIMM installation order
Lockstep mode with memory
mirroring disabled
Lockstep mode with memory
mirroring enabled
1 DIMM connectors 9 and 15 DIMM connectors 1 and 9, and
DIMM connectors 15 and 19
2 DIMM connectors 6 and 24 DIMM connectors 6 and 10, and
DIMM connectors 16 and 24
3 DIMM connectors 1 and 19 DIMM connectors 2 and 8, and
DIMM connectors 14 and 20
4 DIMM connectors 10 and 16 DIMM connectors 5 and 11, and
DIMM connectors 17 and 23
5
DIMM connectors 8 and 14 DIMM connectors 3 and 7, and
DIMM connectors 13 and 21
6 DIMM connectors 5 and 23 DIMM connectors 4 and 12, and
DIMM connectors 18 and 22
7 DIMM connectors 2 and 20
8 DIMM connectors 11 and 17
9 DIMM connectors 7 and 13
10 DIMM connectors 4 and 22
11 DIMM connectors 3 and 21
12 DIMM connectors 12 and 18
58 System x3850 X6 and x3950 X6 Installation and Service Guide
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