Re: chekcpoints, BUFFER settings, and optimization

David Williams <djw@smooth1.demon.co.uk> wrote: >>INFORMIX-OnLine Version 7.22.UC1 -- On-Line -- Up 49 days 04:48:56 -- >>85264 Kbytes >> >>Profile >>dskreads pagreads bufreads %cached dskwrits pagwrits bufwrits %cached >>10363 10367 1405830 99.26 3848 4447 6678 42.38 >> > Why is write caching so poor? Try finding out which tables are being >updated. select * from sysmaster:sysptntab? Who cares? These stats, regarding write caching, are almost useless. ALL writes are cached with online, i.e. every update occurs in memory. All that stat tells you is how many flushes you've done relative to the number of records updated. In order to keep checkpoint times low while maintaining maximum possible READ caching (which DIRECTLY affects performance), you want to keep a relatively steady stream of i/o going on at all times (assuming a brisk OLTP profile). That means a relatively high amount of disk writes, which can drop the write cache. But as long as you are not thrashing the hell out of your disks, who cares? Writes are not blocked from the transaction's perspective, so the impact is general and thus typically diffuse in nature (as opposed to writes occuring during checkpoints, during which time transactions are blocked and thus drastically affected).