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:
- System x3690 X5, type 7147, any model
- System x3690 X5, type 7148, any model
- System x3690 X5, type 7149, any model
- System x3690 X5, type 7192, any model
- System x3850 X5, type 7143, any model
- System x3850 X5, type 7145, any model
- System x3850 X5, type 7146, any model
- System x3850 X5, type 7191, any model
- System x3950 X5, type 7143, any model
- System x3950 X5, type 7145, any model
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:
- 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.
- 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
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