Logical Partitioning (LPAR) allows you to carve up your iSeries server into two or more mostly independent logical servers. Each logical server (i.e., partition) runs its own operating system and software. The only points of inter-dependence between/among logical partitions are the following:
- Some hardware resources are shared, while other hardware resources are divvied up in a semi-temporary way among the partitions.
- One -- and only one -- partition must be the primary partition, or controlling partition. This partition must run OS/400. And if this partition is not running then, none of the other partitions (which are called secondary partitions) can run.
While most folks may already be familiar with the basic concepts of LPAR, many have not taken advantage of it yet. However, any shop that develops its own software and has only one iSeries server (and therefore must do testing on the same machine that runs the production software) would be doing a disservice to its employees and shareholders if LPAR isn't used. With a simple setup, you could configure a separate partition for your test environment (no more accidental data scrambles).
New features you'll want to use
Whether you're new to LPAR or have been using it since its inception, you'll want to make sure you don't let two recently introduced features slip by without noticing. Both were introduced with V5, and they make using LPAR almost mandatory, even if you don't need it.
The first, Virtual LAN, allows you to do same-system, partition-to-partition communications. Until V5, using the OptiConnect software was the only way to do this. And it wasn't cheap, since it was designed to do high-speed, fiber-optic, multi-system communications.
Virtual LAN (along with the ability to partition a single processor on some models) makes LPAR no longer just for the big boys with the big bucks. A modest-sized shop operating on a modest budget should now be able to do LPAR and establish communications between logical partitions. That's because virtual LANs can be used without any additional hardware or software.
How Virtual LAN works
Virtual LAN uses TCP/IP to communicate between logical partitions. An iSeries server can have up to 16 virtual LANs configured. Each virtual LAN operates over its own virtual TCP/IP port. If you want to have two or more logical partitions talk to one another, you open up the same virtual port in each partition. This "assigns" each partition to the same virtual LAN.
When a virtual port is enabled in a logical partition, the system creates a virtual LAN communications port/line for it. Virtual LAN communication ports provide the same function as using a 1GB Ethernet adapter. (Token Ring and 10 and 100MB Ethernet networks are not supported with virtual LAN.)
Once you have configured a virtual LAN and have added logical partitions to it, those partitions can communicate with each other just like over any other TCP/IP network. For example, you could use DDM files or sockets to enable applications in different partitions to communicate with each other.
Figure 1 illustrates how configurable virtual LANs can be. Partitions 1, 2, and 3 are connected through Virtual LAN 1. However, partitions 1, 2, and 3, along with partitions 4 and 5, are also connected through Virtual LAN 2. Partitions 4, 5, 6 and 7 are connected through Virtual LAN 3. And partitions 3 and 7 are connected through Virtual LAN 4. In the final analysis, all partitions are interconnected through one route or the other, and most by two or more routes. Your virtual LANs (or your partitions for that matter) need not be so complex -- the figure just gives you an idea of what is possible.
Dynamic resource allocation arrives
Another major improvement for LPAR in V5 is the dynamic movement of resources -- processors, memory, and interactive performance -- between and among logical partitions, without requiring an IPL or a partition restart.
The catch: You must do IOP-level partitioning, and any partitions that you want to participate in the dynamic movement of resources -- either on the giving side or the receiving side -- must be running at least V5R1 of OS/400. So, for example, if your primary partition is running V5R2, but your secondary partition is running V4R5 for development purposes, you won't be able to take advantage of this feature.
The same is true if you are running V5R2 in your primary partition and Linux in a secondary partition -- you won't be able to swap resources between the two, at least not dynamically. In those cases, you will need to bring down the system, make your resource allocation changes, and restart the system.
Each V5 partition allows you to set minimum and maximum values for each resource that that partition can have. The system will then make sure you don't exceed those limits by doing one of the following:
- Trying to take away too many resources from that partition
- Trying to add too many resources to that partition
About the author:
Ron Turull is editor of Inside Version 5. He has more than 20 years experience programming for and managing AS/400-iSeries systems.
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