This will happen if there is no free buffer in the buffer cache for the session to read a new block into. All buffer gets have been suspended.
This will frequently occur because DBWR is not writing out buffers fast enough.
This could happen when a file was read-only and is now read-write. All the existing buffers associated with the read-only file need to be invalidated since they are not linked to lock elements (needed when mounted parallel (shared)). So cache buffers are not assigned to data block addresses until the invalidation is finished.
The session moved some dirty buffers to the dirty queue and now this dirty queue is full. The dirty queue needs to be written to the file system first. The session will wait on this event and try again to find a free buffer
This also happens after inspecting free buffer inspected buffers. If no free buffer is found, Oracle waits for one second, and then tries to get the buffer again (depends on the context).
Small buffer cache can cause high free buffer waits when large amount of PIOs and LIOs are taking place.
Improve DBWR Throughput