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Home -> Community -> Usenet -> c.d.o.server -> Re: redo log groups vs. rollback segments
On Sun, 28 May 2000 03:43:07 GMT, orauser_at_my-deja.com wrote:
>From the docs I understand that redo logs are used to
>log all changes to the database while rollback segments are
>used to log changes that occur within a transaction?? Is that
>correct? Does that mean that redo logs grow much more than
>rollback segs?
Redo logs are used to allow the database to recover from an instance crash, and to allow you to recover lost data when you lose a datafile. A database's redo log is essentially a record of every operation on the database. When you commit a transaction, Oracle ensures that your comitted changes are written to the redo log before it returns control back to you. If an instance then crashes without recording the most recent comitted changes to the datafiles, Oracle will detect that and recover those changes from the redo log. The redo log is written to redo log files, and if you are running yoru database in ARCHIVELOG mode those files are copied to archive log files.
Rollback segments are used to record before images of data that is changed by a transaction. These before images are used to provide a consistent view (as of a point in time) to other transactions. Data in a rollback segment is not saved after it is not needed for read consistency.
Jonathan
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