LGWR : THE LOG WRITER PROCESS

The LGWR process is responsible for flushing to disk the contents of the redo log buffer located in the SGA. It does this when one of the following is true:

• Every three seconds
• Whenever a commit is issued by any transaction
• When the redo log buffer is one-third full or contains 1MB of buffered data

For these reasons, having an enormous (hundred and thousand megabytes) of redo log buffer is not practical; Oracle will never be able to use it all since it pretty much continuously flushes it.

What is the need of redo log files or LGWR?

The logs are written to with sequential writes as compared to the scattered I/O DBWn must perform .Doing large batch writes like this is much more efficient than doing many scattered writes to various parts of a file. This is one of the main reasons for having a LGWR and redo logs in the first place. The efficiency in just writing out the changed bytes using sequential Input-output outweighs the additional input-output incurred. Oracle could just write database blocks directly to disk when you commit, but that would entail a lot of scattered input-output of full blocks, and this would be significant slower than letting LGWR write the changes out sequentially.