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Cisco UCS C420 Server Installation and Service Guide
OL-27640-01
Chapter 3 Maintaining the Server
Installing or Replacing Server Components
Memory Mirroring Mode
Note
You cannot use memory sparing and memory mirroring at the same time.
When you enable memory mirroring mode, 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.
Memory mirroring reduces the amount of memory available to the operating system by 50 percent
because only one of the two populated channels provides data.
If you choose to enable memory mirroring, you must install sets of identical DIMMs as follows:
•
For a two-CPU system, you must install DIMMs in identical sets of four. Use each set of four
identical DIMMs to populate the sockets in the order shown in
Table 3-4
.
•
For a four-CPU system, you must install DIMMs in identical sets of eight. Use each set of eight
identical DIMMs to populate the DIMM sockets in the order shown in
Table 3-5
.
Lockstep Channel Mode
When you enable lockstep channel mode, each memory access is a 128-bit data access that spans four
channels.
Lockstep channel mode requires that all four memory channels on a CPU must be populated identically
with regards to size and organization. DIMM socket populations within a channel do not have to be
identical but the same DIMM slot location across all four channels must be populated the same.
For example, DIMMs in sockets A1, B1, C1, and D1 must be identical. DIMMs in sockets A2, B2, C2,
and D2 must be identical. However, the A1-B1-C1-D1 DIMMs do not have to be identical with the
A2-B2-C2-D2 DIMMs.
If you choose to enable lockstep channel mode, you must install sets of identical DIMMs as follows:
•
For a two-CPU system, you must install DIMMs in identical sets of four. Use each set of four
identical DIMMs to populate the sockets in the order shown in
Table 3-4
.
•
For a four-CPU system, you must install DIMMs in identical sets of eight. Use each set of eight
identical DIMMs to populate the DIMM sockets in the order shown in
Table 3-5
.
