Ruby Stepansky with IBM North America Lab services was kind enough to remind me of a special DB2 registry variable: DB2_USE_ALTERNATE_PAGE_CLEANING
This variable can be unset (implicitly off) or set to the values OFF or ON. When it is set to ON, DB2 uses a proactive method of page cleaning, writing changed pages to disk, keeping ahead of LSN_GAP, and proactively finding victims. Doing this allows the page cleaners to better utilize available disk I/O bandwidth. Setting this to ON thus cancels out, or negates, any value set for CHNGPGS_THRESH rendering it inconsequential.
Further due diligence on this registry variable shows that it was introduced in V8.1.4, but there was an apar defect which was corrected in V8.1.8 ( V8.1 FP8 ).
If you follow Chris Eaton's blog on ITtoolbox, Chris also blogged about DB2_USE_ALTERNATE_PAGE_CLEANING and provided a helpful query in this blog post.
The bottom line
If you are running DB2 V8 Fixpack 9a or higher, you might consider running this command: db2set DB2_USE_ALTERNATE_PAGE_CLEANING=ON
Use a database snapshot, or your dbsnap2.txt file, to verify the alternate page cleaning method is working well for you. Generally, you should see zeros or small numbers for the snapshot items shown below:
- LSN Gap cleaner triggers = 0
- Dirty page steal cleaner triggers = 0
- Dirty page threshold cleaner triggers = 0
If you see large numbers for any of these values, then DB2_USE_ALTERNATE_PAGE_CLEANING is probably not set, or set to OFF, and the CHNGPGS_THRESH value is probably too high (for example, the default value is 60%).
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