The IBM i Journal Cache Sweeper Knob
May 28, 2014 Joe Hertvik
After publishing my latest article on improving IBM i journal performance with journal caching, IBM Rochester Software Engineer Chad Olstad wrote in with the following comment, which expands on journal caching and offers an alternate view of journal caching’s relationship with commitment control.
“I wanted to point out the one thing you did not mention in the article is the cache sweeper knob, which controls how [old] stale cached journal entries are allowed to be before they are flushed to disk. [The cache sweeper knob] can be customized by the user as of V6R1 via [the Change Journal Attributes command,] CHGJRNA CACHEWAIT(nn), where nn is the number of seconds of journal entries the customer is willing to lose if they were to crash.”
After getting Chad’s email, I checked out the Change Journal Attributes command by typing in CHGJRNA and pressing the F4 key, which produced the following screen.
The Cache wait time (CACHEWAIT) parameter is the journal caching piece that Chad is referring to. CACHEWAIT is the maximum number of seconds that an IBM i system will wait before writing “stale” cached journal entries from memory to disk (flushing the cache). The benefit here is that you can tweak CACHEWAIT to either write cached journal entries to disk at a faster rate (decrease the value) or at a slower rate (increase the value).
The system default value for CACHEWAIT is 30 seconds. But there are some specific rules that you must be aware of for on CACHEWAIT to work.
Chad also added these thoughts on how journal caching relates to commitment control:
“We also thought your comment on commitment control may have been a bit strong. The purpose of commitment control is not to thwart journal caching, but rather to ensure transaction integrity.”
The comment I made in the previous article was that “Commitment control breaks journal caching by putting your job back into the journal wait state that journal caching is designed to avoid,” which as Chad said may have been too strong and unappreciative of what commitment control does for data integrity. If you want to turn on journal caching while using commitment control, I recommend checking with IBM or your business partner to make sure your system is correctly set up for your needs.
Thanks to Chad Olstad for pointing me toward this additional information on journal caching and commitment.
Joe Hertvik is an IBM i subject matter expert (SME) and the owner of Hertvik Business Services, a service company that provides written marketing content and presentation services for the computer industry, including white papers, case studies, and other marketing material. Email Joe for a free quote for any upcoming projects. He also runs a data center for two companies outside Chicago, featuring multiple IBM i ERP systems. Joe is a contributing editor for IT Jungle and has written the Admin Alert column since 2002. Check out his blog where he features practical information for tech users at joehertvik.com.