A Couple Additional Locking Ideas and Thoughts
Before concluding this series, here are a few more guidelines and thoughts on DB2 locking and concurrency:
- It is a good idea to use clustering to encourage
DB2 to maintain data that is accessed together on the same page. If you use
page locking, fewer locks are required to access multiple rows if the rows are
clustered on the same page or pages. You can also use larger page
sizes to control the amount of data locked for page locking.
- Consider using the free space parameters to influence locking. If you increase free space, fewer rows are stored on a single page. Therefore, fewer rows are locked by a single page lock. This approach can decrease contention. However, it consumes additional disk storage, and it can also decrease the performance of table space scans (because more pages with fewer rows must be read). Additionally, keep in mind that as data is added to the table the free space will decrease (because the new data is using it). As such, locking issues may become more prevalent.
- You can also decrease the number of rows per page using the MAXROWS option of the CREATE TABLESPACE statement. The fewer rows per page, the less intrusive page locking will be because fewer rows will be impacted by a page lock. This approach is probably better than the free space approach (previous bullet) because new data will not impact number of rows per page.
- Design your application programs with locking considerations in mind. THis is the Number One thing to remember to increase concurrency and minimize the impact of locking on DB2 application and system performance. You can minimize the effect of locking through proper application program design. This means:
- Limiting the number of rows that are accessed by coding predicates to filter unwanted rows
- Requesting only the data (rows and columns) that your actually need to perform your business processes
- Perform modifications as close to the end of the unit of work as possible
- And remember to avoid bachelor programming syndrome (see Part 9).
Of course, there are probably many more hints, tips, and guidelines for developing DB2 databases and applications with concurrency in mind, but I think a 17 part series is sufficient for my blog. If you want more details on concurrency (or any other aspect of DB2 for z/OS) might I recommend the latest edition of my book -- DB2 Developer's Guide, 6th edition.
And be sure to come back and review this series on locking if you get confused as you work to maximize the concurrency of your DB2 queries, transactions, and programs.
Finally, as a service to my readers, this post includes a directory/index to the 16 separate posts that make up this series.
Index of Blog Posts on DB2 Locking