Lenovo ThinkSystem SR950 Setup Manual page 143

Memory sparing
In memory-sparing mode, one memory rank serves as a spare for other ranks on the same channel in case
they fail. The spare rank is held in reserve and not used as active memory until a failure is indicated, with
reserved capacity subtracted from the total available memory in the system.The DIMM installation order for
memory sparing varies based on the number of processors and memory modules installed in the server.
After an error threshold is surpassed in a system protected by memory sparing, the content of a failing rank
of DIMMs is copied to the spare rank. The failing rank is then taken offline and the spare rank placed online
for use as active memory in place of the failed rank. Since the failover process involves copying of memory
content, the level of memory redundancy provided by memory sparing is less than that provided by memory
mirroring: memory mirroring is the preferred failure-protection choice for critical applications.
Figure 26. Processor and memory module layout
Memory sparing guidelines:
• The spare rank must have identical or larger memory capacity than all of the other active memory ranks on
the same channel.
• If installing DIMMs that are one rank, follow the population sequences listed below.
• If installing DIMMs with more than one rank, follow population sequences specified for independent
memory mode. See "Independent memory mode" on page 63.
The memory sparing DIMM population sequences for each supported processor configuration are:
• "Installation order: memory sparing with two processors" on page 140
• "Installation order: memory sparing with three processors" on page 141
• "Installation order: memory sparing with four processors" on page 145
• "Installation order: memory sparing with six processors" on page 149
• "Installation order: memory sparing with eight processors" on page 161
For information about configuring memory settings, including how to enable memory sparing, see "Memory
configuration" in the ThinkSystem SR950 Setup Guide.
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Chapter 3 Server hardware setup
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