oracle 11g log buffer和redo buffer值不同

                   数据库版本
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
PL/SQL Release 11.2.0.4.0 - Production
CORE 11.2.0.4.0 Production
TNS for Linux: Version 11.2.0.4.0 - Production
NLSRTL Version 11.2.0.4.0 - Production
redo buffer值
SQL> show sga
Total System Global Area  780824576 bytes
Fixed Size      2257312 bytes
Variable Size    452988512 bytes
Database Buffers   322961408 bytes
Redo Buffers      2617344 bytes

log buffer值
SQL> show parameter log_buffer
NAME         TYPE  VALUE
------------------------------------ ----------- ------------------------------
log_buffer        integer  2437120


这两个值怎么不一样呢？