Divide and conquer

Why buy a new server when you can split your existing machine? As Nick Langley explains, partitioning is no longer just a...

Why buy a new server when you can split your existing machine? As Nick Langley explains, partitioning is no longer just a mainframe tool.

Among the many potential benefits of logical partitioning (Lpar), the one that's received most attention in recent years is server consolidation. It's really a no-brainer; you take the workload off several medium-to-large systems, or a lot of small ones, and move it to a larger machine, where each application or group of applications gets a partition of its own.

You can, if you like, create a virtual version of the original server within a partition. The new virtual server then has access to its 'own' processor(s), storage, i/o and memory, although it may be sharing them with other virtual servers.

So far, such consolidation has usually been within a particular range of platforms - from multiple small iSeries to a large one, for example. But with the growing availability of Linux on large Unix systems and the mainframe, it's become much easier to poach other suppliers' customers, as IBM has recently done at the Scandinavian telco Telia.

Fewer systems means lower hardware costs and, in theory, lower software costs - though this will depend on licensing policies, as we'll see later. Maintenance and administration costs come down, and you may get by with a smaller IT team, using one management console. However, by eliminating under-used capacity, you set yourself new tuning and load-balancing challenges. Partitioning technology is being progressively automated, to handle the work of allocating and changing partition sizes dynamically. But somebody still has to set the whole thing up, and write the scripts to deal with new applications and priorities.

Logical partitioning was originally developed for IBM mainframes - the S/360, in 1976. It was implemented on the AS/400 two years ago, and IBM promises that it will be available on the p- and xSeries in future.

By around 1997, Hitachi, Amdahl and Sun had their own versions. Within the last year, Hewlett-Packard and Compaq have joined them, and Unisys and Microsoft are working on a version for Windows 2000 Datacenter Server.

Although, according to IBM's own figures, a partition can have 10 per cent lower performance than an equivalent-sized standalone machine, there are other benefits relating to performance and reliability. Applications that conflict for resources can be isolated from one another. Backup partitions can be set up to enable 'granular fail over' for applications.

You can also run old versions of operating systems and applications while migrating to new ones, or set up test versions in partitions, where they won't interfere with the production systems.

Current interest in Lpar has two main business drivers. One is mergers and acquisitions. While new integrated systems are being developed, the existing systems of the merged companies can be run in different partitions.

The second driver is e-business, with its unpredictable workloads. Provided the machine has the capacity, partitions can be enlarged - or set up to enlarge themselves - as demand rises; conversely, capacity can be taken away as demand falls.

Partition sizes can also be scheduled to deal with different levels of traffic at different times of day. As office systems usage drops off at the end of the day, for example, e-shopping enquiries may grow. Partitions can be set up for different languages and time zones, with resources being reallocated as offices open and close around the world.

Partitioning can also be used by web hosting companies to set up virtual servers for their customers. Ensim's ServerXchange, used by around 60 service providers, allows them to put several customers on one machine without degrading performance.

Applications like ERP, with multi-tiered architectures, can be allocated different partitions on a single server. Data warehousing and data mining can be speeded up, because traffic between partitions is handled by the systems bus, rather than a connection between two machines. A partition-aware implementation of OptiConnect, IBM's standard clustering interconnect for AS/400 systems, allows partitions to communicate using the system bus as a high-bandwidth, low-latency channel, enabling applications to be clustered between partitions.

LPar on the iSeries is considerably more sophisticated than anything IBM's competitors can offer. Customers can move as little as one-hundredth of a processor between OS/400 partitions, and partitions can be set up using just one-tenth of a processor. Even a single processor iSeries can have up to four partitions, while Sun and HP only offer partitioning on their top of the range systems.

Sun was the first supplier to implement partitioning on Unix machines. Sun Enterprise 10000 (Starfire) servers can run up 16 partitions, known as Dynamic System Domains (DSD). DSDs are logically isolated from each other. Each uses one or more system boards, containing CPU, memory, I/O, boot-disk and network resources. A domain may be as small as a single board or as large as all boards within the system. DSDs allow resources to be allocated on the fly, without rebooting the system or disrupting work in other partitions. DSDs should be available for systems lower down the range later this year.

Other mid range systems suppliers are catching up. HP's 9000 L Class, N Class and Superdome servers can support one virtual server per CPU, each running its own copy of HP-UX, with the ability to add or remove processors dynamically. Compaq's AlphaServer GS Series servers, running Tru64 Unix or OpenVMS, support logical partitioning.

Microsoft has rather been left behind - its multiple server offerings have concentrated on clustering, in which it also lags the Unix suppliers - but its work with Unisys will provide partitioning on the Unisys ES7000, and ultimately for the entire Windows and Intel-based enterprise server market.

Penguin meets dinosaur
IBM is in the lead in putting Linux into partitions. iSeries will support up to 32 Linux partitions. That may help ensure a future for the iSeries-AS/400, and the ability to run Linux in a partition may help do the same for the mainframe. Early reports indicate that this is easier than you might expect; the German company RWE Energie AG says it installed SuSE Linux on its own Lpar after just 90 minutes, at an event titled 'Penguin meets Dinosaur'.

However, some analysts believe that IBM fell down badly, when it failed to capitalise on the opportunity partitioning presented for 'fairer' mainframe software licensing. When Amdahl and Hitachi brought out their partitioning offerings, they made licensing a key part of their message.

Hitachi's VSF (Virtual Server Facility) allows users to be charged according to the number of processors used by each licensed software product, rather than the system's overall capacity. VSF enables Hitachi mainframe clients to define up to 15 virtual servers, setting precise boundaries within each processor, and assigning unique serial numbers. Computer Associates, BMC, Candle and Compuware are among the ISVs which have endorsed the VSF approach to licensing.

'IBM has lost control of software pricing on System/390 mainframes,' analyst Phil Payne, from Sheffield based Isham, said at the time of the VSF launch. 'Amdahl's Multiple Server Feature (MSF) and HDS's VSF give them exactly what they need; a way to know that users are not deriving more benefit from using their products than those users are paying for. IBM is now at a disadvantage. Not only does its hardware not offer a comparable feature, but its middleware must struggle against ISV products whose suppliers do not insist on licences being paid for capacity unavailable to the application.' Now Payne says it's too early to say whether IBM's forthcoming Workload Licence Charges (WLC) will stop the rot - particularly since the Licence Manager won't be available until late this year. 'For the present, all of my calculations have shown that consolidating multiple G5/G6 servers onto fewer G7 servers actually increases software costs.'

Linux implementation
Linux on the S/390 can be implemented in one of three ways:

  • It can run as the primary OS;
  • The S/390 can be partitioned into up to 15 logical partitions, each of which can run its own OS, including Linux;
  • Customers can run Linux as one of multiple virtual machines in a S/390 using VM/ESA Guest Support.

Using VM, IBM has ousted Sun at Telia, where a S/390 mainframe running Linux is replacing 70 Sun servers hosting customers web sites. Initially the mainframe will provide 1500 customers with their own virtual Linux servers, although the mainframe can support up to 30,000. Telia claims it will now take five minutes to install a new server, instead of five hours.

IBM set the lead in logical partitioning, and has kept it, enabling users to allocate one-hundredth of each processor where competitors deal only in whole CPUs. Lpar is set to spread to all IBM's platforms, and takes in low end as well as the very biggest models. So, great technology; but shame about the licensing.

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