Hardware options installation 43
Memory board 1
configuration
Memory board 2
configuration
Bank A
1A and 5A
Bank B
2B and 6B
Bank C
3C and 7C
Bank D
4D and 8D
1 X
—
—
—
—
—
—
—
2 X
X
—
—
—
—
—
—
3 X
X
X
—
—
—
—
—
4 X
X
X
X
—
—
—
—
5 X
X
X
X
X
—
—
—
6 X
X
X
X
X
X
—
—
7 X
X
X
X
X
X
X
—
8 X
X
X
X
X
X
X
X
Online spare memory
Online spare memory provides protection against degrading FBDIMMs by reducing the likelihood of
uncorrectable memory errors. This protection is available without any operating system support.
An understanding of single-rank and dual-rank FBDIMMs is required to understand memory usage in
online spare mode. FBDIMMs can either be single-rank or dual-rank. Certain FBDIMM configuration
requirements are based on these classifications. A dual-rank FBDIMM is similar to having two single-rank
FBDIMMs on the same module. Although only a single FBDIMM module, a dual-rank FBDIMM acts as two
separate FBDIMMs. The purpose of dual-rank FBDIMMs is to provide the largest capacity FBDIMM for the
current DRAM technology. If the current DRAM technology allows for 2-GB single-rank FBDIMMs, a dual-
rank FBDIMM using the same technology would be 4-GB.
In online spare mode, a single rank of memory acts as the spare memory. For single-rank FBDIMMs, the
entire FBDIMM acts as the spare memory. For a dual-rank FBDIMM, only half of the FBDIMM acts as the
spare memory while the other half is available for operating system and application usage.
If one of the non-spare FBDIMMs receives correctable memory errors at a higher rate than a specific
threshold, the server automatically copies the memory contents of the degraded rank to the online spare
rank. The server then deactivates the failing rank and automatically switches over to the online spare.
Because FBDIMMs that experience a high rate of correctable memory errors also have a higher
probability of receiving an uncorrectable memory error, this configuration reduces the likelihood of
uncorrectable memory errors that would result in server downtime.
Online spare is performed per memory board. For dual memory board configurations, two ranks are
used for online spare memory. One memory board can fail over to the associated online spare while the
other memory board is still protected.
In addition to general configuration requirements ("FBDIMM installation guidelines" on page 47), online
spare memory has the following FBDIMM configuration requirements:
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