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Architecting Power Systems for I/O

Architecting I/O

The left side of the drawer (from the rear) is P1 and the right side is P2. Adapters should be split across the fanouts for redundancy. For consistency, I like to split them to the same slot on each side. For example, if I’m using P1-C1 for one of my fibre pairs then I will use P2-C1 for the other. The PCIe adapter placement guide provides the list of slot priorities for the EMX0 I/O drawer.

5887 EXP24S Disk I/O Drawer

If you also have an EXP24S disk I/O drawer this adds a level of complication. If it is a simple mode 1 installation (all disks owned by one LPAR) then it is most likely connected using one or two adapters (some of the adapters must be installed in pairs). The EJ0L and EJ14 adapters are only supported in four slots – 9, 7, 4, 2 of the CEC and only 9 and 7 are available in a two module system. They’re also supported in the EMX0 PCIe3 adapter drawer.

When selecting the EXP24s you need to specify disk mode. Disk modes refer to how the disk plane is split up and connected and must be selected at order time. If the disk is mode 2 or 4 then additional adapters are required. The EXP24S has 24 disk bays in groups of 6. In mode 1 all 24 disks are connected to one (or a pair of RAID) adapter/s. In mode 2 they are broken into two sets of 12 and in mode 4 there are four sets of six disks. The drawer can support up to four LPARs if it’s in mode 4, but at least four (and possibly eight) adapters are needed along with any required cables. Each increase in mode requires more adapters to support the additional connections. These adapters can be in the CEC or in the EMX0 PCIe drawer.

Pre and Post Order

Even if it’s not mandatory I always do a solutions assurance (now called TDA). The intent is to ensure the configuration is valid and will meet the needs of the workloads. Part of that TDA includes a spreadsheet that maps out the location of every adapter card. That spreadsheet can then be used to install the adapters in the predetermined slots prior to loading the operating system. That way everything is set to the best possible options right from the start.


When trying to plan for I/O on POWER servers it is important to refer to the Technical Overview and Adapter Placement guides to ensure the configuration being built is valid and will meet the needs for the workloads. As an example, if you need to have 6 LPARs boot from internal disk then you will need a split backplane in the CEC and a mode 4 EXP24S disk drawer. This will most likely require one or more EMX0 PCIe3 adapter drawers. It is much easier to get this right when ordering than to try and fix it after the server arrives.

This level of planning isn’t onerous but should be done for all new servers in order to get the most out of the new equipment. Although we used the E850C as an example, the adapter guides and overview manuals exist for all the current servers and can be found with a simple Google search. It is well worth investing the time and effort in this level of planning as it allows you to maximize performance and redundancy while also ensuring the solution will work.


E850C 8408-e4e Adapter Placement Guide


E850C 8408-e4e Technical Overview and Introduction

Care and feeding of VIO servers

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