RE: Long Transaction
Comments embedded >===== Original Message From "Obnoxio The Clown" <obnoxio@hotmail.com> ===== >From: Edward Rosenthal <edrosenthal@home.com> >> >>Er....I have the distinctive aroma of cow pats in my nose...where does the >>physical log get retrieved, exactly? >>i love the image. anyway >>i'm from petaluma, calif. and we have plenty of cows. > >I'm sure you do. But isn't it illegal? > >>i was thinking, and correct me if i'm wrong, (i'm sure you will!) >>that each time the buffer is modified there has to be an io write to the > >Well, not quite. Every time a previously unmodified buffer is modified. If >the buffer was already "dirty", it doesn't get rewritten to the physical >log. > >>physical log >>to keep the before image handy, for a rollback(?). this comes from the >>buffer >>from >>the read cache ...as each buffer modification takes place the physical log >>is >>keeping before image rows by having the cache write to it. >>when the 75% is reached,or a checkpoint, the physlog is written to the >>database, > >Well, no. The physical log isn't written to the database at all. It's only >used for rollback. Once the flush of modified buffers is complete, the >physical log is simply truncated. I thought the physical log was only used for fast recovery. If it was used for rollbacks, wouldnt it have to be alot bigger to cover some of those larger transactions? > >>and flushed. >>and because the next buffer modification needs the next before image, >>the physical log needs to have the before image written to from the >>database... >>hence, the "retrieval"... > >Right. I've never heard a write described as a retrieval before. My mistake, >I'm sure. >________________________________________________________________________ >Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com Will ------------------------------------------------------------ This e-mail has been sent to you courtesy of OperaMail, as a free service from Opera Software, makers of the award-winning Web Browser, Opera. Visit us at http://www.opera.com/ or our portal at: http://www.myopera.com/ Your free e-mail account is waiting at: http://www.operamail.com/ ------------------------------------------------------------