Free Buffer Wait

Possible Causes:

•          This means we are waiting for a free buffer but there are none available in the cache because there are too many dirty buffers in the cache.
•          Either the buffer cache is too small or the DBWR is slow in writing modified buffers to disk.
•          DBWR is unable to keep up to the write requests.
•          Checkpoints happening too fast – maybe due to high database activity and under-sized online redo log files.
•          Large sorts and full table scans are filling the cache with modified blocks faster than the DBWR is able to write to disk.
•          If the number of dirty buffers that need to be written to disk is larger than the number that DBWR can write per batch, then these waits can be observed.

Actions:

•          Reduce checkpoint frequency.
•          Increase the size of the online redo log files .
•          Examine the size of the buffer cache 

-consider increasing the size of the buffer cache in the SGA .

•          Set disk_asynch_io = true set.
•          If not using asynchronous I/O increase the number of db writer processes or dbwr slaves.
•          Ensure hot spots do not exist by spreading datafiles over disks and disk controllers.
•          Pre-sorting or reorganizing data can help.

Remarks:

•          Understanding and Tuning Buffer Cache and DBWR Note# 62172.1
•          How to Identify a Hot Block within the database Buffer Cache. Note# 163424.1