Cisco Systems UCUCSEZC220M3S manual DIMM Sparing and Rank Sparing, Memory Mirroring, 3-21, per CPU

Chapter 3 Maintaining the Server

Installing or Replacing Server Components

DIMM Sparing and Rank Sparing

DIMM and rank sparing can be enabled in the BIOS configuration utility.

Sparing involves utilizing one of the DIMM pairs or rank pairs within each memory riser as a spare unit. When any of the other DIMM pairs within the same memory riser experiences errors beyond a pre-defined threshold, it fails over to the spare DIMM pair. Spared DIMMs and ranks are hidden from the user and the OS so that the BIOS can migrate to a spare unit when it finds degrading DIMMs.

When sparing is enabled, the available system memory is lesser than the total installed memory.

•When using DIMM sparing, the available memory equals total installed memory minus the size of spared DIMMs.

•When using rank sparing, available memory equals total installed memory minus the size of the spared ranks. Rank size equals DIMM size divided by the number of ranks.

Memory Mirroring

When memory mirroring is enabled, the memory subsystem simultaneously writes identical data to two channels. If a memory read from one of the channels returns incorrect data due to an uncorrectable memory error, the system automatically retrieves the data from the other channel. A transient or soft error in one channel does not affect the mirrored data, and operation continues unless there is a simultaneous error in exactly the same location on a DIMM and its mirrored DIMM. Memory mirroring reduces the amount of memory available to the operating system by 50% because only one of the two populated channels provides data.

If you choose to enable memory mirroring, populate the DIMM slots in the order shown in Table 3-5.

Table 3-5	Memory Mirroring DIMM Population
Number of DIMMs
per CPU		CPU 1 Population	CPU2 Population
4		A1, B1; A2, B2	E1, F1; E2, F2
6		A1, B1; C1, D1; A2; B2	E1, F1; G1, H1; E2, F2
8		A1, B1; C1, D1; A2, B2; C2, D2	E1, F1; G1, H1; E2, F2; G2, H2

	Cisco UCS C220 Server Installation and Service Guide
OL-25760-01		3-21