The LOG_BUFFER size will be set by default, by Oracle internal algorithm.
In 10G R2, Oracle combines fixed SGA area and redo buffer [log buffer] together.
If there is a free space after Oracle puts the combined buffers into a granule, that space is added to the redo buffer. Thus you see redo buffer has more space than expected. This is an expected behavior.
Bug 4930608 documents this :
"In 10.2 the log buffer is rounded up to use the rest of the granule...
The granule size can be found from the hidden parameter "_ksmg_granule_size" "
In 10G the log_buffer could fall between the log_buffer specified in spfile and the granule size.
A minimum value can be specified, but Oracle can initialize it with a bigger value, as decided by its internal algorithm.
metalink: 604351.1
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