Memory mirroring mode installation order

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 server.

Memory mirroring guidelines:

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.
Each DIMM must be identical in size and architecture.
DIMMs on each memory channel must be of equal density.
If two memory channels have DIMMs, mirroring occurs across two DIMMs (channels 0/1 will both contain the primary/secondary memory caches).
If three memory channels have DIMMs, mirroring occurs across all three DIMMs (channels 0/1, channels 1/2, and channels 2/0 will all contain primary/secondary memory caches).

With two processors

The following table shows the DIMM population sequence for memory mirroring when two processors are installed.

Table 1. Memory Mirroring with two processors
Total	Processor 1												Processor 2												Total
DIMMs	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	DIMMs
8*			3		5			8		10					15		17			20		22			8
12*	1		3		5			8		10		12	13		15		17			20		22		24	12
24*	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	24

Note

DIMM configurations with an asterisk (*) listed in the table support the Sub NUMA Clustering (SNC) feature, which can be enabled via UEFI. SNC is not supported if DIMM population does not follow the sequence indicated by the table above.

With four processors

The following table shows the DIMM population sequence for memory mirroring mode when four processors are installed.

Table 2. Memory mirroring with four processors
Total	Processor 1												Processor 2												Total
DIMMs	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	DIMMs
16*			3		5			8		10					15		17			20		22			16
24*	1		3		5			8		10		12	13		15		17			20		22		24	24
48*	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	48
Total	Processor 3												Processor 4												Total
DIMMs	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41	42	43	44	45	46	47	48	DIMMs
16*			27		29			32		34					39		41			44		46			16
24*	25		27		29			32		34		46	37		39		41			44		46		48	24
48*	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	10	41	42	43	44	45	46	47	48	48

Note

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Memory mirroring mode installation order

With two processors​

With four processors​

With two processors

With four processors