IBM DDIMM Memory
What a DDIMM is, why IBM buffers memory on Power instead of wiring DDR directly to the processor, and the DDIMM slot count and capacity range on every Power 11 model.
A DDIMM is a Differential DIMM: IBM's buffered memory module, used across every Power 10 and Power 11 system. It is not a standard DDR5 DIMM with a different label. The buffer chip on the module is what lets IBM decouple memory from the processor's pin budget, and that single design choice is why a two-socket Power 11 system can address 8 TB where a conventional DDR-attached server of the same socket count cannot come close.
Why IBM buffers memory when nobody else does
Conventional servers wire DDR channels straight from the memory controller to the DIMM slots in parallel. That works, but every channel costs processor pins, and pins are finite. The Open Memory Interface replaces those parallel buses with a high-speed serial protocol and puts the DDR controller on the module itself. The processor talks OMI; the DDIMM talks DDR to its own chips.
More capacity per socket
Serial signaling uses far fewer pins per port, so a socket can support up to 32 DDR5 ports where conventional wiring manages 8 to 12. Independent architectural analysis puts the resulting improvement at three to four times the memory bandwidth and capacity of direct-attach DDR.
Media independence
Because the DDR controller lives on the module, IBM can change memory generation without redesigning the processor. That is how the same OMI interface carried DDR4 DDIMMs on early Power 10 and carries DDR5 DDIMMs on Power 11.
Lane failure recovery
OMI can reroute around a failed memory lane rather than failing the channel. On a machine expected to run for years without a maintenance window, that is a large part of the reliability case.
The trade
DDIMMs are IBM parts, not commodity DIMMs. You cannot populate a Power 11 with third-party DDR5, and secondary-market pricing behaves accordingly. Budget memory as an IBM line item, not as a generic component.
DDIMM slots and capacity by Power 11 model
DDIMM configuration across the Power 11 line
| Model | Machine type | DDIMM slots | Memory range |
|---|---|---|---|
| S1112 | 9242-21B / 21T | 4 | 512 GB maximum. 256 GB usable for AIX, Linux and VIOS; 64 GB for IBM i at P05 |
| S1122 | 9824-22A | 32 | 64 GB to 4 TB |
| S1124 | 9824-42A | 32 | 64 GB to 8 TB |
| L1122 | 9856-22H | 32 | 64 GB to 4 TB |
| L1124 | 9856-42H | 32 | 64 GB to 8 TB |
| E1150 | 9043-MRU | 32 per node | 256 GB to 16 TB per node |
| E1180 | 9080-HEU | 64 per node | Up to 64 TB across up to 4 nodes |
The S1112 is the outlier and the one that catches buyers. Its 512 GB platform ceiling is not the number that matters for an IBM i workload, because IBM i on that machine is bounded at 64 GB regardless of what the chassis holds.
Slot count and maximum capacity are separate constraints. A partly populated 32-slot system has room to grow without a processor change; a fully populated one needs larger DDIMMs, which means replacing modules rather than adding them. Check current population before assuming an upgrade is additive.
How to see what DDIMMs are installed
From IBM i
WRKHDWRSC *CMN and the system service tools hardware listing show installed memory resources. WRKSYSSTS gives you what is actually being used, which is the number that tells you whether more is warranted.
From AIX, Linux or VIOS
lscfg -vp lists the memory modules with part numbers and locations, which is what a supplier needs to quote matching parts. prtconf reports total installed memory.
From the HMC
Managed system properties, memory tab, shows installed against assigned memory and which slots are populated. This is the fastest way to see whether an upgrade means adding modules or replacing them.
Before ordering
DDIMM feature codes are model-specific. Confirm the machine type, the current slot population and the existing module capacity together, because mixing capacities across a channel is not always supported.
Related
The processor side of the same design is covered on the Power 11 CPU page, and full memory ceilings per model are on the Power 11 specs page. The memory listing in the directory is at DDR5 DDIMM memory for Power 11, and the short definition is at DDIMM in the glossary.