free buffer waits

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