16. What is Redolog buffer? What exactly it stores?

A log buffer is a circular buffer in the SGA that holds information about changes made to the database. This information is stored in the redo entries. Redo entries contain the information necessary to reconstruct or redo changes made to the database by insert, update, delete, create, alter, or drop operations. Redo entries are primarily used for database recovery as necessary.

The server processes generate redo data into the log buffer as they make changes to the data blocks in the buffer. LGWR subsequently writes entries from the redo log buffer to the online redo log.

The redo log buffer is a RAM area (defined by the initialization parameter log_buffer) that works to save changes to data, in case something fails and Oracle has to put it back into its original state (a ?rollback?). When Oracle SQL updates a table (a process called Data Manipulation Language, or DML), redo images are created and stored in the redo log buffer. Since RAM is faster than disk, this makes the storage of redo very fast.