IBM Support

Memory error (Not Configured message) after using DIMMs of mixed size and rank in the same memory channel - IBM System x

Troubleshooting


Resolving The Problem

Source

RETAIN tip: H212714

Symptom

An IBM System x model X5 system cannot correctly configure the memory when populated with mixed DIMM ranks or sizes in the same memory channel, under some specific combinations (for example, 16 GB Dual-Rank with 4 GB Quad-Rank).

(where DIMM = Dual In-Line Memory Module)

In the UEFI F1 Setup menu, users see that these mixed memory DIMMs are Not Configured.

Affected configurations

The system can be any of the following IBM servers:

This tip is not software specific.

This tip is not option specific.

Solution

This is a permanent restriction.

Workaround

Do not configure the memory where mixed rank/size memory DIMMs are in the same memory channel at the same time.

For example:

  Slot  Size and Rank    Memory Channel

1     8GB 2Rank    (memory channel 0, pair A)
2     4GB 4Rank    (memory channel 0, pair B)
3     empty        (memory channel 1)
4     empty        (memory channel 1)
5     empty        (memory channel 2)
6     empty        (memory channel 2)
7     4GB 4Rank    (memory channel 3, pair B)
8     8GB 2Rank    (memory channel 3, pair A)

The above memory channel map applies to x3850 x5 and x3690 x5. Every eight (8) memory slots are grouped as one (1) unit, with four (4) channels.

For the x3850 x5, this applies to slots 1-8 on each memory card.

For the x3960 x5, this applies to slots 1-8 and 9-16 on the system system board, and slots 17-24 and 25-32 on the memory mezzanine card.

Additional information

Two memory detection rules in UEFI code contain potential conflicts:

  1. Mixing DIMM ranks in the same memory channel is permitted as long as the largest rank is installed first. DIMM pairs must be the same.
  2. Mixing DIMM sizes in the same memory channel is permitted as long as the largest size is installed first. DIMM pairs must be the same.

When a DIMM of larger size or smaller rank is installed with a DIMM with smaller size or larger rank, conflicts occur.

Document Location

Worldwide


Operating System

System x:Operating system independent / None


Applicable countries and regions

  • Worldwide
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Document Information

More support for:
System x->System x3850 X5->7146

Document number:
864720

Modified date:
30 January 2019

UID

ibm1MIGR-5095582

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