Memory Sparing And Mirroring; Figure 8. Four Dimm Configuration Example - Intel Server Board S5000PAL User Manual

In non-mirrored mode, all DIMMs with the same slot number within a given branch must
match (size, technology, manufacturer). It is not required to match DIMMs between
different slot numbers.
DIMMs must meet the following requirements:
•
Use only Fully Buffered DIMMs (FBD) with DDR2 DRAM technology.
•
Use only FBD DDR2-533 and FBD DDR2-667 stacked DIMM modules.
In determining your memory requirements, the need for memory sparing or memory
mirroring must be considered. For a complete list of supported memory DIMMs, see the
links under

Memory Sparing and Mirroring

The chipset includes hardware that supports memory mirroring and memory on-line
sparing. Both memory mirroring and memory on-line sparing provide a way to prevent
data loss in case a DIMM fails.
With memory mirroring the system maintains two copies of all data in the memory
subsystem. If a DIMM fails, the data is not lost because the second copy of the data is
available from the mirrored DIMM in the opposite channel. The system will not fail due
to memory error unless both the primary and the mirrored copy of the data become corrupt
at the same time.
In a mirrored system, the maximum usable memory is one-half of the installed memory,
with a minimum of four DIMMs installed. Since the data is duplicated across DIMMs, it
means that up to one-half of the installed DIMMs are actively in use at any one time. The
remaining DIMMs are used for mirroring.
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Figure 8. Four DIMM Configuration Example

"Additional Information and Software."
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Intel Server Board S5000PAL User's Guide