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Appendix B: Memory configuration options
168
Memory sparing
Sparing allows for memory to be set aside to replace memory under
use when a DIMM’s correctable error count has reached a specified
threshold. Spared memory configurations do not provide redundant
copies of memory and the system cannot continue to operate when an
uncorrectable error occurs.
The DIMMs on each memory board must be installed in pairs. Each pair
is referred to as a bank. A bank may consist of one rank (a pair of
single-sided DIMMs) or two ranks (a pair of double-sided DIMMs). You
can refer to “Installing DIMMs” on page 73 for instructions on how to
install a DIMM on a memory board.
When the memory mode is set to maximum performance or maximum
compatibility, the BIOS setup supports setting one rank aside to server
as spare for each memory board. When the correctable error rate for a
failing rank exceeds the error threshold for switching to spare, the
contents of the failing rank are copied to the spare rank. At the
completion of the copy, the failing rank is disabled and the spare rank
is used in its place. The BIOS reports the failing rank with a SEL (system
event log) event, updates the DIMM error LED on the memory board,
and sends memory RAS (reliability, availability, and serviceability)
commands to the BMC (baseboard management controller) to update
the system memory state. The DIMMs with the failed rank are disabled
on subsequent boots. The spare rank is no longer used for spare, but
instead used as a system memory.
When the BIOS setup is configured for sparing, the largest rank is
chosen to serve as the spare. This ensures that the contents of any
failing rank fit on the spare rank. The amount of the available memory
in the system is reduced by the size of the spare rank. If only one rank
is available on a memory board, the system BIOS does not configure
this rank as a spare.
The BIOS setup utility executes a memory test prior to configuring the
memory in POST and when a memory board is inserted into the system
during a memory hot-removal or hot-add operation.
To configure memory sparing:
1 Do either option below:
• Run BIOS setup. See “Entering BIOS setup” on page 96.
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