Lenovo System x3850 X6 Quick Start Manual page 11

Important notes
Keep in mind the following important aspects of the memory subsystem when setting up and
using your server:
The optimal memory configuration is to install DIMMs in quantities of eight per CPU. This
configuration ensures that every channel on every memory buffer is populated.
Additionally, it ensures that no matter what memory mode is chosen, the system functions
correctly.
Proper memory population rules for DIMMs include:
– Higher capacity (ranked) DIMMs must be installed first. Follow the population
sequence for the appropriate mode.
– The server supports a maximum of eight ranks (octal-rank) per DDR3 channel. Note
that LR-DIMMs might exceed eight ranks per channel through rank multiplication.
– The server supports 1.35-volt (low-voltage) registered DIMMs, depending on the
memory configuration settings in the Setup utility. The server can also operate at
1.5-volt.
– RDIMMs and LR-DIMMs cannot be mixed in the same system. Make sure to populate
the largest DIMMs in the channel first to enable optimal performance.
• RDIMMs are available in 4 GB, 8 GB, and 16 GB.
• LR-DIMMs are available in 32 GB and 64 GB.
– A minimum of one DIMM must be installed for each Compute Book. Depending on the
memory mode (for example, Lockstep), this configuration might not be sufficient for
operation.
The default memory mode for the X6 system is Lockstep, which is important for those
scenarios where less than the optimal number of DIMMs per processor are installed.
Always be sure to properly configure the memory mode in Unified Extensible Firmware
Interface (UEFI) before changing the DIMM installation order, but prior to a reboot, for this
configuration to take effect.
When not fully populating system memory according to preferred practices, changing the
memory mode can cause the system to fail to boot. The memory population label on the
CPU book is for Performance mode. Lockstep mode DIMMs should be installed in pairs.
You can use the Advanced Settings Utility™ (ASU) tool to remotely flip the memory mode.
For example:
asu64 Memory.MemoryMode Independent OR ./asu64 Memory.MemoryMode Lockstep
The maximum operating speed of the server's memory is determined by the slowest
DIMM in the server.
If swapping out a failed DIMM, make sure you move the DIMM to a different SMI and
re-enable the failed DIMM slot.
If planning to install eXFlash DIMMs, note that eXFlash DIMMs are RDIMMs and,
therefore, cannot be used with either the 32 GB or 64 GB LR-DIMMs.
No more than one eXFlash DIMM per channel is supported, and at least one RDIMM must
already be present in the same channel as the eXFlash DIMM that it will be installed in.
Lenovo System x3850 X6 and x3950 X6 Quick Start Guide
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