IBM 7378, Problem Determination And Service Manual

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Memory mirroring mode
Memory-mirroring mode replicates and stores data on two pairs of DIMMs within
two channels simultaneously. If a failure occurs, the memory controller switches
from the primary pair of memory DIMMs to the backup pair of DIMMs. You must
enable memory mirroring through the Setup utility. For details about enabling
memory mirroring, see “Starting the Setup utility” on page 303. When you use the
memory-mirroring feature, consider the following information:
v
When you use memory mirroring, you must install a pair of DIMMs at a time.
One DIMM must be in channel 0, and the mirroring DIMM must be in the same
connector in channel 1. The two DIMMs in each pair must be identical in size,
type, rank (single or dual), and organization. They do not have to be identical in
speed. The channels run at the speed of the slowest DIMM in any of the
channels. See Table 13 on page 229 for the DIMM connectors that are in each
pair.
v Channel 2, DIMM connectors 7, 8, 15, and 16 are not used in memory-mirroring
mode.
v
The maximum amount of available memory is reduced to half of the amount of
installed memory when memory mirroring is enabled. For example, if you install
64 GB of memory, only 32 GB of addressable memory is available when you use
memory mirroring.
The following illustration shows the memory channel interface layout with the DIMM
installation sequence for memory mirroring mode. The numbers within the boxes
indicate the DIMM population sequence in pairs within the channels, and the
numbers next to the boxes indicate the DIMM connectors within the channels. For
example, the following illustration shows that the first pair of DIMMs (indicated by 1s
inside the boxes) should be installed in DIMM connector 3 on channel 0 and DIMM
connector 6 on channel 1. DIMM connectors 7, 8, 15, and 16 on channel 2 are not
used in memory-mirroring mode.
The following table lists the DIMM connectors on each memory channel.
Table 12. Connectors on each memory channel
Memory channel DIMM connectors
Channel 0 1, 2, 3, 9, 10, 11
Channel 1 4, 5, 6, 12, 13, 14
Channel 2 (not used in memory mirroring) 7, 8, 15, 16
Figure 11. Memory channel interface layout
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IBM System x3400 M3 Types 7378 and 7379: Problem Determination and Service Guide