regarding concept of physical logging??

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 -- Posted via CNET Help.com http://www.help.com/