Log buffer

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Hi,
On 11.2.0.4 on AIX

I have:

SQL> show parameter log_buffer

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
log_buffer                           integer     10485760

But when I run:

SQL> show sga

Total System Global Area 2137886720 bytes
Fixed Size                  2248080 bytes
Variable Size            1157628528 bytes
Database Buffers          956301312 bytes
Redo Buffers               21708800 bytes

Why log_buffer  value is different from Redo Buffers value? Aren't they the same buffer?

Thanks

[ErmanArslansOracleBlog]

LOG_BUFFER specifies the amount of memory (in bytes) that Oracle uses when buffering redo entries to a redo log file.
Redo Buffers that you see in show sga output may be different (higher) the value of LOG_BUFFER and this is normal.
We have extra pages there.. for instance the guard pages for protecting the log buffer..

Check-> Init.ora Parameter "LOG_BUFFER" Reference Note (Doc ID 30753.1)
There you will see the following ->

he memory set aside for the log buffer may also be surrounded by guard pages to help protect the redo buffer, hence the size of Redo Buffers shown at instance startup may differ from the value of LOG_BUFFER set in the init.ora file.  

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Thank you.