Log buffer 的 Latch

rwq_

The redo allocation latch is acquired to allocate memory space in the log buffer. Before Oracle9.2, the redo allocation latch is unique and thus serializes the writing of entries to the log buffer cache of the SGA. In Oracle 9.2. Entreprise Edition, the number of redo allocation latches is determined by init.ora LOG_PARALLELISM.   The redo allocation latch allocates space in the log buffer cache for each transaction entry.  If transactions are small, or if there is only one CPU on the server, then the redo allocation latch also copies the transaction data into the log buffer cache. If a logswitch is needed to get free space this latch is released as well with the redo copy latch.