DBWR lru scans: the number of times that the database writer scans the lru
for more buffers to write. The scan can be invoked either by a make free
request or by a checkpoint.

DBWR summed scan depth: can be divided by DBWR lru scans to determine the
length of the scans through the buffer cache. This is NOT the number of
buffers scanned. if the write batch is filled and a write takes place
to disk, the scan depth halts.

DBWR buffers scanned: is the total number of buffers scanned when looking
for dirty buffers to write to disk and create free space. The count
includes both dirty and clean buffers. It does NOT halt like the
DBWR summed scan depth.

When a process searches DB_BLOCK_MAX_SCAN_CNT buffers in the list
without finding a free buffer. If this is the case it signals the
DBWR to write to make way for free buffers

_db_writer_scan_depth
"Number of LRU buffers for dbwr to scan when looking for dirty buffers"