One of most interesting inquiries I received last month from a Giga client was a question about key criteria for deciding between data warehousing on DB2/6000 and S/390. Here are the main points of the reply.
The greatest advantage of DB2 S/390 is in reliability, availability and serviceability. Although single image alternative platforms such as AIX- RS/6000 continue to improve, they are still not as bulletproof as the mainframe. Of course, there is a high availability cluster multiprocessing (HACMP) option (using a form of clustering) for AIX. That will at least partly address the reliability and availability issues. Notice also that the initial cost advantage of AIX is thereby reduced.
Data warehousing systems start out being less than mission-critical and requiring a service-level agreement (SLA) consistent with, for example, 12x6 availability (twelve hours a day, six days a week). However, everything seems to slide in the direction of 24x7; and soon the IT staff has to manage a different, more demanding SLA with infrastructure designed for a less rigorous scenario. Obviously, that spells trouble. Whatever the agreement you reach, be sure to document it thoroughly (in writing) and make sure everyone has a copy.
Do you have the capacity the CPU cycles, the storage, the processing window on the mainframe? If you have unused capacity CPU cycles and available storage or either one on the mainframe, then it may actually be less expensive to use the mainframe. With the partitioning and workload management provided on S/390 (now IBM's zSeries), dedicating a portion of a S/390 environment to a data warehouse is doable. In case you must perform capacity planning and schedule a mainframe upgrade, that will impact cost and schedule significantly. Other-wise, the new AIX system will have an advantage if it is standalone and can be brought in independently of an existing platform.
What is the available storage technology? If you already have system managed storage (SMS) on the mainframe, the lives of the DBAs and IT staff should be made more simple with fewer outages due to mismanaging storage allocation due to policy-base storage management. SMS does not, in itself, prevent running out of space (how could it?); but, it reduces the risk of running out of space due to staff oversight in having manually to monitor hundreds or thousands of data structures. Depending on how many data structures are implied by managing 500GB to 1TB of storage, storage technology becomes the critical path. Policy-managed storage is not available on AIX, although what used to be called AdStar Storage Management (ADSM) and is now a part of the Tivoli storage technology solution simulates some of the features.
If you have 1TB of user data, not including indexes, aggregates, etc., then you could end up needing multi-terabytes of storage and a correspondingly higher price point. If you think the data volume is going to grow significantly beyond 1TB or CPU cycles ramp up quickly in the face of long-running, complex queries, and assuming you wish to remain IBM-centric, you may want to look at IBM SP hardware using DB2 UDB (extended enterprise edition). This is the share-nothing architecture which is required for very large systems at the multiterabyte level. Regardless of the underlying platform, we are talking mainframe pricing here, are we not? (Examples of pricing can be found under TPC-H on the Transaction Processing Council Web site at www.tpc.org.)
What and where are the skills of the DBA staff? If they are familiar with DB2 UDB utilities on AIX, then that is a significant advantage for AIX (or vice versa). Managing backup, recovery and reorganization requires an understanding of data integrity in a variety of forms. DB2 log archiving and managing of those logs for system recoverability is (relatively speaking) a piece of cake on S/390. Supplementary storage technology is required on AIX see the earlier paragraph about ADSM. DB2 S/390 has supported online reorganization since version 5, so 24x7 availability is feasible from a database scheduling perspective. However, DB2 on AIX/NT does not support online reorganization and will not do so until at least the next release. So, that is another availability advantage for S/390. In other respects, ease of use (usability) is what you already know. There are now good third-party tools available for database monitoring and fast utilities on non-MVS platforms. However, the mainframe is still the most heavily instrumented and instrumentable platform at a corresponding cost.
Since most mainframes are, in fact, maxed-out at 100 percent utilization running transactional workloads, maybe the performance advantage is indeed with the single image AIX box dedicated to its task. However, that is a function of a given environment, not of the intrinsic capabilities of the processor configurations. In short, clients should prepare to load up on main memory and allocate large volumes of buffers to DB2 (or any database, for that matter) regardless of the underlying platform. Undertake some role-playing and be the data, be the electron, in the wire. I/O is the slowest path through the system and the fastest I/O is one that never occurs because the data is already cached in the buffer.
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