Big Blue to Kick iSeries Partitioning Up a Notch with V5R2

by Timothy Prickett Morgan

With the initial release of partitioning on the AS/400 server, only Apache 6XX and 7XX; Northstar 7XX; and I-Star and S-Star 8XX machines with two or more processors could support logical partitions. The initial logical partitions in the AS/400 line, with the 1999 debut of OS/400 V4R4 partitions, supported one logical partition per processor on 6XX, SXX, and 7XX iron. These were static partitions, and any change in them required customers to reboot their AS/400 servers. With V4R5, IBM supported different release levels within partitions and added some resiliency and scripting features to make partitions more useful. With OS/400 V5R1, IBM allowed a single processor to be carved up into four virtual partitions on iSeries Model 270, 820, and 830 servers with eight or fewer processors; Model 840 machines (which have either 12 or 24 processors) topped out at 32 partitions. V5R1 also saw the introduction of the Virtual LAN communications scheme, which uses the TCP/IP protocol, communicating over the internal iSeries system bus, to create a high-speed virtual network that runs at memory speeds for inter-partition communication.

Logically, with four partitions per processor, a top-end Model 840-2420 should have been able to handle 96 partitions. My guess is that the microcode controlling OS/400 queues and symmetric multiprocessing, on which the logical partitioning features partly rely, was limited to 32 processors. In queuing theory, 32 is a magic number, so it could be some other limit. I think the 32 partition limit had to do with the virtual LAN internal partition-to-partition communications software that IBM implemented with OS/400 V4R5. It looks like IBM has extended this virtual LAN to support 255 simultaneous virtual LANs, which means it can support 255 distinct logical partitions. This is all armchair architecture on my part, of course. But if the virtual LAN wasn't the limiting factor, the machines would be able to support 256 partitions. With TCP/IP networking, 255 is a magic number, and I think it might have something to do with TCP/IP subnet masking that IBM is using in the virtual LAN feature of OS/400.

While my sources are pretty confident that up to 255 logical partitions will be available with OS/400 V5R2, they do not know the limitations on this expanded partition support. IBM could allow that many partitions on any iSeries Regatta server, or it could limit the partitioning based on the physical number of Power4 processors in a machine. For instance, IBM could say that up to 16 shared-processor logical partitions can be used for each Power4 chip, whether it has a single Power4 core or a dual Power4 core (see the related pSeries and iSeries Regatta story in this issue for more on IBM's possible Power4 midrange server lineup). These limitations may have more to do with marketing and customer expectations than anything else. It is also unclear if this significantly expanded logical partitioning will be backcasted into other iSeries machines that do not use the Power4 processor--particularly S-Star models, which are still fairly new. IBM may have added electronics and microcode to the iSeries Regattas that enables support for a maximum of 255 partitions, and it is very unlikely that it will backcast this support into prior generations of machines. Shared-processor partitioning was available only on S-Star machines, you'll remember, because of changes IBM made in the guts of these machines.

That said, changes in microcode may allow S-Star machines to support a larger number of logical partitions. If IBM is indeed able to support approximately 16 partitions per processor because of changes in OS/400's microcode, then a current 24-way Model 840 could, in theory, get support for a whopping 384 partitions, but would have to max out at the 255 partitions because of the supposed virtual LAN limits. IBM could also limit the number of partitions on any machine to the maximum number of external LAN cards that can be attached to a server, which would enable each partition to have at least one real LAN link to the outside world. In this case, a Model 840-2420 would have 96 partitions, a Model 830 would have 72 partitions, a Model 820 would have 30 partitions, and a Model 270 would have 8 partitions. Of course, IBM could do something a lot different from this--including pushing off larger numbers of logical partitions for OS/400 servers until the debut of OS/400 V5R3, scheduled tentatively for late 2003.