Re: regarding concept of physical logging??

sudhakar wrote: > > hello, > i have a serious confusion regarding tha concept of physical logging, > this might sound silly for an expert kindly bear with me and explain.. > > according to what the book says.. > physical logbuffer contains the before images of the data that taken from > disk for modification.. > and its main use is for restoring the physical consistency of tha data.. > i.e data is copied from disk to the shared memory buffer and a copy is > made to the physical log buffer. so the physical log buffer will contain > value before modification..if the system crashes physical log is flushed > to disk so that it contains old data.. > > my doubt? > if only a copy is made from disk to shared memory buffer pool. the > disk will be havin the old data. why again flush it from physical logs.. > > this is really confusin > i know i am conceptually gettin locked some where > someone pls unlock me > bye > sudhakar UNLOCK sudhakar Syntax error! UNLOCK TABLE sudhakar Table sudhakar not found! Erm...., let me try, but this usually takes a 5 day course before people start to get it... :-) The physical log is an area on disk containing before images as you say. The physical log buffer is just a cache used to speed up writes to the physical log. The fast recovery process will copy the pages in the physical log back to the buffer pool and perform a checkpoint, which gives you a consistent set of data back on disk. The disk may "be havin the old data", as you put it, but it may also "be havin some old and some new data". The fast recovery process ensures a consistent set of data. Of course all you REALLY need to know is, "does it work?", and the answer is "yes".... most of the time. ;-) Cheers, -- Mark. +----------------------------------------------------------+-----------+ | Mark D. Stock mailto:mdstock@mydas.freeserve.co.uk |//////// /| | http://www.informix.com http://www.informixhandbook.com |///// / //| | http://www.iiug.org +-----------------------------------+//// / ///| | |This email will self-destruct in |/// / ////| | |10 sec. If you received this email |// / /////| | |in error, sorry about the mess. |/ ////////| +----------------------+-----------------------------------+-----------+