The Log Buffer

It is impossible to generalise, but if there is such a thing as a 'typical' Oracle database, you might measure the size of the Shared Pool and the Buffer Cache in the several hundred megabytes range, or bigger. In comparison, most Log Buffers anywhere in the World would not be much bigger than, tops, 10M or so.

The Log Buffer is thus a relatively tiny area of memory in which a record of the changes you make to data is stored. Every insert, update or delete that you perform will generate a record of what row was affected and how it was modified. That record is written, in the first instance, to the Log Buffer component of the SGA. From there it is periodically saved to special files on disk, thus making the record of what transactions have taken place on a database permanent.

The transaction record is generically known as redo, and we can therefore say that redo is first written (for reasons of speed) into the Log Buffer of the SGA for each and every transaction that users perform.

As the name implies, redo is generated in order to let us 'do again' those transactions which might otherwise have been lost because of hard disk failure or power supply failure or software error or user stuff-up. Redo, in short, is Oracle's recovery mechanism, and the Log Buffer is simply the first place redo is created.