User`s manual

26 Server Board Features
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. 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
four DIMMs are actively in use at any one time. If eight 2 GB DIMMs are installed, the maximum
usable memory is 8 GB. The remaining four 2 GB DIMMs are used for mirroring.
Three methods for memory mirroring are available:
• Four identical DIMMs are used in DIMM sockets 1A, 2A, 1B, and 2B. If the DIMM in socket 1B
fails, the DIMM in socket 2A takes over. If the DIMM in socket 1A fails, the DIMM in socket
2B takes over. See Figure 16.
• Six DIMMs are used with identical DIMMs in sockets 1A and 1B and with identical DIMMs in
sockets with identical DIMMs in sockets 2A, 2B, 3A, and 3B. See Figure 17.
• If the DIMM in socket 1A fails, the DIMM in socket 1B takes over.
• If the DIMM in socket 2B fails, the DIMM in socket 3A takes over.
• If the DIMM in socket 2A fails, the DIMM in socket 3B takes over.
• Eight DIMM population with identical devices in DIMM Bank 1 and Bank 2, and identical
devices in Bank 3, and Bank 4. DIMMs 1A, 1B, 2A, 2B must be identical and DIMMs 3A, 3B,
4A, 4B must be identical. See Figure 18.
• If the DIMM in socket 1B fails, the DIMM in socket 2A takes over.
• If the DIMM in socket 1A fails, the DIMM in socket 2B takes over.
• If the DIMM in socket 3B fails, the DIMM in socket 4A takes over.
• If the DIMM in socket 3A fails, the DIMM in socket 4B takes over.
a
Figure 16. Four DIMM Memory Mirroring
empty empty mirror primary