After I installed I/O Express, there was a decrease in system performance

There are several things which could contribute to a decrease in system performance.

CPU - By doing a MONITOR SYSTEM, you can see how much CPU is being utilized. If you notice over a period of time that the CPU is running at or above 90%, then your system is CPU-bound during that period of time. Depending on the balance between your work load and CPU usage, you may find that your system is CPU-bound during most, or all, of the day. Once a system has become CPU-bound, everything runs much slower. It is important to understand that I/O Express will continue to function properly, regardless of CPU status, and it allows your system to run more efficiently by not having to wait for many of the read I/Os to complete from disk. Therefore, the system can do more work, which uses more CPU. So, if you have an already saturated system, I/O Express may appear to slow your system, when it has actually helped the system run with fewer read I/O interruptions.

READ I/Os - There are two kinds of read I/Os: repetitive reads and non-repetitive reads. Repetitive reads are continually reading the same information over and over again. Non-repetitive reads are reading different information all the time, as in image backups. The only kind of read I/O which will benefit a system with I/O Express is repetitive reads. It is important to understand that I/O Express caches data blocks; therefore, anything which can be read can be cached, including VMS code, file headers, text files, application code, data base files, etc. So every system will get some benefit running I/O Express.

MEMORY - When you do a SHOW MEMORY, you can determine how many free pages there are on the Free Page List, at the moment you issued the command. If you notice that there never seems to be much more than 256 pages in cache, and that the amount of available pages on the Free Page List is less than the "Min Free List before use" field when you run IOX-STATS, then you are MEMORY BOUND. I/O Express was designed to run on systems with very little memory; however, in order to get the most benefit from I/O Express, you do need to have plenty of memory. You may find that your free list fluctuates to the extremes most of the time. In this situation, I/O Express is continually having to trim its cache, and then add more pages to its cache over and over again. This thrashing effect, which is caused by a lack of available memory on the system and/or the work load, could cause a decrease in system performance during those periods. It is important to understand that regardless of your system configuration, I/O Express will work with the system and its resources.

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