Re: FAQ

>Date: Thu, 28 Jul 1994 15:16:11 -0500 >From: cwakins@jabba.rmc.com (Clem W. Akins) >Subject: FAQ >X-Informix-List-Id: <list.4420> > >Regarding the FAQ, here are some criticisms and comments: > >In 6.1, Configuring OnLine: >default Transaction high-water marks -- Should be less than 50%, to allow > a roll-back. Have enough logs that whatever number you choose is > enough for your largest long transaction. 70/80 is not nearly > low enough. On what do you base this assertion? With a reasonable log configuration (say more than 4 logs), the 70/80 limit is very unlikely to run you into trouble. Unless you are mass inserting or deleting extremely small records (say 1-8 bytes, though I'm tempted to suggest 1-4 bytes), it is very difficult to run into problems. I do not recall seeing systems run into trouble with 80/90 figures which are the default in tbconfig.std. I tend to leave them parameters at their default, even on production machines, though I have also lowered it to 80/70 when users request it. I suspect that there is some granularity built into the testing of the log space left; if you only have 3 logs, then the logs are only 67% full when you get to the last log, so maybe you could run into problems with LTXHWM > 67. Ensuring you have enough log space in enough logs (8-16 for a production system) means that one log represents 6.25-12.5%, and gives ample safety margin with 70/80. Reducing these parameters to less than 50% is not something I'd recommend; there is definitely an upper-limit on how much log space you can require to do a rollback, because the CLM records (which are the cause of the trouble) are of finite size (16 bytes, if memory serves me right), and there is at most on CLM for each operation performed by the user, and each operation has a 12 byte header plus some data, such as the whole record which is inserted or deleted, or both the before and after image for an updated record. Unless these records are tiny, the amount of space used to generate the LTX dwarfs the amount of space needed to rollback. I suppose that if you have multiple users who regularly and simultaneously start operations which become LTX transactions, then you may have problems. But that is at least partly a question of education, and at least partly a question of operational controls on the OnLine system. I consider that you have to be trying quite hard to run into trouble with LTXHWM = 70, LTXEHWM = 80. I also admit that the problems which occur if this is wrong are so severe that erring on the side of caution is advisable. Yours, Jonathan Leffler (johnl@informix.com) #include <disclaimer.h>