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Free buffer waits occur when when a server process was unable to find a free buffer and has asked DBWR to make a free buffer by writing out dirty buffers.
The remedy for Free Buffer Waits depends on the type of block. This can be obtained through the V$WAITSTAT view. If the block is a segment header, it could be a freelist contention problem. If the block is an undo header or undo block, rollback segments could be too small or too few (if not using an undo tablespace). If the block is a data block and all of your SQL is well tuned, you might consider increasing the size of the buffer cache. In addition, write times at the OS level should be examined since DBWR may be running slow due to I/O issues. Consider increasing DB_WRITER_PROCESSES or DBWR_IO_SLAVES.