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Chapter 3. IBM System x3850 X5 and x3950 X5
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Table 3-19 MAX5 memory mirroring setup
3.8.5 Memory sparing
Sparing provides a degree of redundancy in the memory subsystem, but not to the extent of
mirroring. For more information regarding memory sparing, see “Memory sparing” on
page 29. Use these guidelines for installing memory for use with sparing. The two sparing
options are DIMM sparing and rank sparing:
DIMM sparing
Two unused DIMMs are spared per memory card. These DIMMs must have the same rank
and capacity as the largest DIMMs that we are sparing. The total size of the two unused
DIMMs for sparing is subtracted from the usable capacity that is presented to the
operating system. DIMM sparing is applied on all memory cards in the system.
Rank sparing
Two ranks per memory card are configured as spares. The ranks have to be as large as
the rank relative to the highest capacity DIMM that we are sparing. The total size of the
two unused ranks for sparing is subtracted from the usable capacity that is presented to
the operating system. Rank sparing is applied on all memory cards in the system.
These options are configured by using the UEFI during boot.
3.8.6 Effect on performance by using mirroring or sparing
To understand the effect on performance by selecting various memory modes, we use a
system that is configured with X7560 processors and populated with sixty-four 4 GB
quad-rank DIMMs.
Figure 3-21 on page 90 shows the peak system-level memory throughput for various memory
modes measured using an IBM-internal memory load generation tool. There is a 50%
decrease in peak memory throughput when going from a normal (non-mirrored) memory
configuration to a mirrored memory configuration.
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