Monitoring the Redo Log Buffer (Oracle)

The redo log buffer is the part of the System Global Area (SGA) that holds information about changes made to the database. Each of these changes generates a 'redo entry'. Redo entries are needed to reconstruct these changes during the recovery process.

The Database Monitor displays the following information on the redo log buffer:

Size of the Redo Log Buffer (Size)

The default size for the Oracle redo log buffer on SAP systems is 320 KB. It is set by the init.ora initialization parameter log_buffer ( LOG_BUFFER (Oracle)). This setting is normally adequate for most installations.

Allocation Retries (Allocation retries/Alloc fault rate)

Allocation retries shows the number of failed attempts to allocate space in the redo log buffer. A value greater than zero normally indicates that the Oracle log writer process (LGWR) could not write redo entries from the buffer to disk (in the online redo log files) immediately, but had to wait for a redo log file switch to perform this action.

If the number of allocation retries constantly increases during normal database operation, you may need to increase the redo log buffer.

Note the following: Larger online redo log files reduce the number of redo log file switches since more data is stored in each file. If you then perform high volume insert operations (common in table reorganization or data loading), large amounts of redo entries will be generated making allocation errors more likely.

Alloc fault rate shows the ratio of allocation retries to total redo entries in the redo log buffer since database instance startup. Error rates of more than 1% under normal operating conditions should be investigated.