Checkpoint Stall problem?
Whilst searching for information on DS_HASHSIZE, I found the following, I went in search of the results and summary on CDI but couldn't find it. Two questions arise: 1) Which versions does this apply to? and 2) Does anyone know where the results are located? A search on CDI archive for Checkpoint Stall returned 54 pages of results!!!!!!! Many thanks Colin Dawson www.ladbrokes.com The Checkpoint Stall Problem We are all aware that updates must wait while a checkpoint completes, and that SELECTs are not affected. This is not quite true! Read on. If you have more than 500 dirty buffers at checkpoint time, the checkpoint thread will not relinquish control of CPU VP#1 until all buffers have been handed off to CLEANER threads-as long as there are page cleaner threads available that are not busy. Since user threads currently executing in CPU VP#1 are not migrated to other VPs prior to beginning the checkpoint, or you may only have one CPU VP, this means that even FETCHs and OPENs will hang for approximately the first half of the checkpoint duration (longer if you set LRU_MAXDIRTY and LRU_MINDIRTY high). If you have response-time requirements measured in seconds, this can be deadly. If no page cleaner thread is ready, then the checkpoint thread will relinquish the VP and place itself on a wait queue until a cleaner thread frees up. With less than 500 dirty buffers, a single cleaner is launched to handle all dirty buffers and the checkpoint thread relinquishes the CPU VP to other threads after that launch. This sounds better, but the result is the same. Based on this, I am recommending that sites with large numbers of buffers and short response time requirements configure with one or only a few page cleaners so that the checkpoint will have to enter a wait state while the previous page cleaners complete their last assignments. This should cause the checkpoint thread to release the VP and permit queries to continue. This issue is being examined by Informix R&D and, by the time this goes to press, I should have had an opportunity to test the latest theoretical solution. Search the C.D.I. archives at http://www.iiug.org/ for my results. sending to informix-list