Chapter 3. IBM System x3850 X5 and x3950 X5 89Table 3-19 MAX5 memory mirroring setup3.8.5 Memory sparingSparing provides a degree of redundancy in the memory subsystem, but not to the extent ofmirroring. For more information regarding memory sparing, see “Memory sparing” onpage 29. Use these guidelines for installing memory for use with sparing. The two sparingoptions are DIMM sparing and rank sparing: DIMM sparingTwo unused DIMMs are spared per memory card. These DIMMs must have the same rankand capacity as the largest DIMMs that we are sparing. The total size of the two unusedDIMMs for sparing is subtracted from the usable capacity that is presented to theoperating system. DIMM sparing is applied on all memory cards in the system. Rank sparingTwo ranks per memory card are configured as spares. The ranks have to be as large asthe rank relative to the highest capacity DIMM that we are sparing. The total size of thetwo unused ranks for sparing is subtracted from the usable capacity that is presented tothe 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 sparingTo understand the effect on performance by selecting various memory modes, we use asystem that is configured with X7560 processors and populated with sixty-four 4 GBquad-rank DIMMs.Figure 3-21 on page 90 shows the peak system-level memory throughput for various memorymodes 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) memoryconfiguration to a mirrored memory configuration.Number of DIMMs MAX5DIMM 1DIMM 2DIMM 3DIMM 4DIMM 5DIMM 6DIMM 7DIMM 8DIMM 9DIMM 10DIMM 11DIMM 12DIMM 13DIMM 14DIMM 15DIMM 16DIMM 17DIMM 18DIMM 19DIMM 20DIMM 21DIMM 22DIMM 23DIMM 24DIMM 25DIMM 26DIMM 27DIMM 28DIMM 29DIMM 30DIMM 31DIMM 324 x x x x8 x x x x x x x x12 x x x x x x x x x x x x16 x x x x x x x x x x x x x x x x20 x x x x x x x x x x x x x x x x x x x x24 x x x x x x x x x x x x x x x x x x x x x x x x28 x x x x x x x x x x x x x x x x x x x x x x x x x x x x32 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
