Redolog Buffer Cache : SGA Component of Oracle

The redo log buffer is a circular buffer in the SGA that stores redo entries describing changes made to the database.
A redo record is a data structure that contains the information necessary to reconstruct, or redo, changes made to the database by DML or DDL operations. Database recovery applies redo entries to data files to reconstruct lost changes.The database processes copy redo entries from the user memory space to the redo log buffer in the SGA. The redo entries take up continuous, sequential space in the buffer. The background process log writer process (LGWR) writes the redo log buffer to the active online redo log group on disk.

LGWR writes redo sequentially to disk while DBW performs scattered writes of data blocks to disk. Scattered writes tend to be much slower than sequential writes. Because LGWR enable users to avoid waiting for DBW to complete its slow writes, the database delivers better performance.

The LOG_BUFFER initialization parameter specifies the amount of memory that Oracle Database uses when buffering redo entries. Unlike other SGA components, the redo log buffer and fixed SGA buffer do not divide memory into granules.

Redolog Latches 

When a change to a data block needs to be done, it requires to create a redo record in the redolog buffer executing the following steps:

* Ensure that no other processes has generated a higher SCN
* Find for space available to write the redo record. If there is no space available then the LGWR must write to disk or issue a log switch
* Allocate the space needed in the redo log buffer
* Copy the redo record to the log buffer and link it to the appropriate structures for recovery purposes.

The database has three redo latches to handle this process:

Redo Copy latch

The redo copy latch is acquired for the whole duration of the process described above. The init.ora LOG_SIMULTANEOUS_COPIES determines the number of redo copy latches. It is only released when a log switch is generated to release free space and re-acquired once the log switch ends.

Redo  allocation latch
The redo allocation latch is acquired to allocate memory space in the log buffer. Before Oracle9.2, the redo allocation latch is unique and thus serializes the writing of entries to the log buffer cache of the SGA. In Oracle 9.2. Enterprise Edition, the number of redo allocation latches is determined by init.ora LOG_PARALLELISM.   The redo allocation latch allocates space in the log buffer cache for each transaction entry.  If transactions are small, or if there is only one CPU on the server, then the redo allocation latch also copies the transaction data into the log buffer cache. If a log switch is needed to get free space this latch is released as well with the redo copy latch.

Redo writing latch
This unique latch prevent multiple processes posting the LGWR  process requesting log switch simultaneously. A process that needs free space must acquire the latch before of deciding whether to post the LGWR to perform a write, execute a log switch or just wait.