LOG_BUFFER specifies the amount of memory, in bytes, that Oracle uses when buffering redo entries to a redo log file. Redo log entries contain a record of the changes that have been made to the database block buffers. The LGWR process writes redo log entries from the log buffer to a redo log file.
In general, larger values for LOG_BUFFER reduce redo log file I/O, particularly if transactions are long or numerous.
If the 'redo log space requests' statistic in V$SYSSTAT is non-zero, you should increase LOG BUFFER.
SELECT name, value
FROM V$SYSSTAT where name ='redo log space requests';
A good starting point for the log_buffer is 64K, there is generally no advantage in making the log_buffer larger than 1 Mb.
VALUE: Maximum is 512 KB or 128 KB * CPU_COUNT, whichever is greater.