Parameter LOG_BUFFER

Question

Hello guys,
a litte question about parameter LOG_BUFFER.
I thought all the time, that the modified data are immediately written to the redo log files by the LGWR, if a value is changed. But in the documentation for parameter LOG_BUFFER stands:
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. In a busy system, a value 65536 or higher is reasonable.
http://saturn.uab.es/server.920/a96536/ch1100.htm
So i think, if the instance and db crashed and the redo log entries are cashed in the sga (and not written immediately to redo files), they are lost and no recovery can be done.
Or do i missunderstood the documentation about that parameter? But then how can be I/O reduced, if the data is written immediately to redolog files....
Thanks
Regards
Stefan

Answer 1

Ahh ok ...
But a last information about the log_buffer:
Here is a statement from an articel:
LGWR will clean up buffers if log_buffer 1/3 full, 1mb log_buffer full or commit occurs. If we change log_buffer to 10mb will it helpful to improve performance.
Ok now ... the log_buffer is in the sga.. and i activated Automatic Memory Management (sga_target is set)
Now i saw this strange thing...
Oracle AMM only sizes the follwing parameters SID.* (example values from my database) ... the sga_target and log_buffer i was setting by hand:
SID.__db_cache_size=2785017856
SID.__java_pool_size=16777216
SID.__large_pool_size=16777216
SID.__shared_pool_size=838860800
SID.__streams_pool_size=0
*.sga_target=3670016000
*.log_buffer=1048576
I set log_buffer to 1048576 in my spfile... but if i make "show parameter" ... i get the following:
SQL> show parameter log_buffer
NAME TYPE VALUE
log_buffer integer 14254080
So Oracle sizes the log_buffer with AMM ... but didn't write it down to the spfile like the other caches (db_cache_size, java_pool_size, and so on...)
Kind of strange...
Regards
Stefan