In the last installment, we discussed some home-brewed techniques and utilities for identifying and balancing loads across the disk units that make up an ASP. These techniques and utilities were somewhat labor-intensive and imperfect. However, until V4R4 it was about the best we could do without getting into some really heavy-duty programming.
In V4R4 (OK, so they are not so new), IBM introduced a group of ASP load-balancing commands. If you are interested in high performance, you need to use these commands. The best part is they are exceptionally easy to use and can be run while your system is active.
Quick load balancing
The STRASPBAL command (Start ASP Balance) starts a background process that balances the loads on the ASPs you specify. If you just want to do a quick balancing job to make sure all disk units in each ASP have the same percentage of used and unused space, simply execute the following command and then carry on with your normal activities:
STRASPBAL ASP(*ALL) TYPE(*CAPACITY) TIMLMT(*NOMAX)
Using the STRASPBAL command this way is also a good way to quickly distribute data within an ASP after adding new disk units to that ASP.
Note: ASP balancing is done in the background so, as mentioned, you can just start it and forget it. Also, the balancing process will not run over on IPL; when the system is shut down, it also will be shut down. However, just start it again using the same command as before, and it will continue where it left off.
Load balancing to improve performance
Not that the method just discussed won't improve performance. It most likely will, but you'll be trading much performance for time and convenience. But if you invest a little extra time and work, you can really improve performance. Here's how to do it.
The first thing to do is use the TRCASPBAL command (Trace ASP Balance) to start the process that traces ASP balancing information. The following command is an example:
TRCASPBAL ASP(*ALL) SET(*ON)
The ASP balance tracing is also done in the background. And like ASP balancing, the tracing cannot run across an IPL-boundary, but it can be restarted after an IPL and it will continue adding trace information to that previously gathered (as long as you don't clear the trace data in between using the *CLEAR value for the SET parameter on the TRCASPBAL command).
Once the trace has run long enough -- and "long enough" depends on your system and how it is used -- you can balance your ASPs for ultimate performance using either the usage balancing or hierarchical storage management balancing.
Use the usage balancing option (*USAGE on the TYPE parameter) if all or most of the disk units in each ASP have roughly the same performance characteristics (e.g., all disk units in an ASP on the same model). Usage balancing tries to improve access time (i.e., seek time, plus latency) by redistributing high-use and low-use data among the drives. First, each drive is given equal percentages of high-use and low-use data. Then, the high-use data is copied to the area on the hard drive with the best access time.
Use hierarchical storage management balancing when your ASPs are configured with disk units that have varying performance characteristics, especially if they vary greatly. Hierarchical storage management balancing redistributes the data in the ASP so the high-use data is on the drives with the best performance and the low-use data is on the drives with the least performance.
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.
- Balance your disk usage for optimum performance
The iSeries does a better job of balancing and managing disk usage than probably any other machine in the industry, but there are times when things get a little out of sync. As of V4R4, IBM has provided a solution that will address several disk performance issues.
- Complete cleanup for your iSeries system
Are you interested in reducing your ASP utilization percentage, but don't know where to start? Here's a list of commands/functions you can perform on your iSeries system to cleanup all the garbage.