Re: Informix to Oracle Migration

"rkusenet" <usenet.rk@gmail.com> wrote in message news:3ci8i4F6juf62U1@individual.net... > In chapter 5 (redo and rollback) he mentions how in some other RDBMS > the transaction log contains both undo and redo information. I know > SQL Server and Informix follow that model. I am not able to pull out > the page number, but to paraphrase him got it. It is actually Chapter 4 Transactions - Page 153 "many other databases treat the log files as transaction logs. they do not have this separation of redo and undo - they keep both in the same file. For those systems, the act of rolling back can be disastrous - the rollback process must read the logs their log writer is trying to write to. They introduce contention into the part of the system that can least stand it. Oracle's goal is to make it so that logs are written sequentially, and no one ever reads them while being written- ever". Note the clever way of exaggerating the operation of rollback as if in any production instance rollback is a regular business operation, worthy of being considered in any design for I/O contention.