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Cisco UCS C24 Server Installation and Service Guide
OL-26647-01
Chapter 3 Maintaining the Server
Installing or Replacing Server Components
• Although 1600 MHz DIMMs can be run in Power Savings Mode (1.35 V operation), 1600 MHz
operation is supported only when the DDR mode is set to Performance Mode (see “
Enabling
Low-Voltage DIMM Operation.”) A 1600 MHz DIMM set to Power Savings Mode operates at
1066 MHz.
• Observe the DIMM mixing rules shown in Table 3-3.
• Memory mirroring reduces the amount of memory available by 50% because only one of the two
populated channels provides data. When memory mirroring is enabled, DIMMs must be installed in
sets of four as described in
Memory Mirroring, page 3-23.
• Note the following restrictions when using UDIMMs. Even though a UDIMM might be rated at
1600 MHz, actual operational speeds are slower because of the Intel implementation.
–
In Performance Mode (1.5 V operation), UDIMMs run at 1333 MHz in 1 DPC configurations
or at 1066 MHz in 2 DPC configurations.
–
In Power Saving Mode (1.35 V operation), UDIMMs run at 1333 MHz in 1 DPC configurations
or at 1066 MHz in 2 DPC configurations.
Enabling Low-Voltage DIMM Operation
You can enable low voltage (1.35 V) DIMM operation for all DIMMs in the server. There is a setting in
the BIOS Setup utility that you can use to change the DDR memory mode to Power Saving mode, as
described in the following procedure:
Step 1 Enter the BIOS setup utility by pressing the F2 key when prompted during bootup.
Step 2 Select the Advanced tab.
Step 3 Select Low Voltage DDR Mode.
Step 4 In the pop-up window, select either Power Saving or Performance Mode:
• Power Saving Mode–Prioritizes low-voltage memory operation.
• Performance Mode–Prioritizes performance memory operation. If you mix low-voltage DIMMs
with standard DIMMs, the system defaults to this setting.
Step 5 Press F10 to save your changes and exit the setup utility.
Ta b l e 3-3 DIMM Mixing Rules
DIMM Parameter Mixed Mix Across Single Channel? Mix Across Multiple Channels?
DIMM size (4, 8, 16 GB) No—Must be same size in
channel.
Yes—Can be different sizes in server.
DIMM speed (1333 or
1600 MHz)
No—Must be same speed in
channel.
No—Must be the same speed in server.
DIMM type (RDIMM or
UDIMM)
No—Must be same type in
channel.
No—Must be the same type in server.