Oracle Core: Essential Internals for DBAs and Developers

Database Writer

Oracle uses the redo log and lgwr to make transactions durable, so we don’t need to copy changed database buffers from memory to disk the instant we change them; but it’s nice (and in almost all cases necessary) to copy changed buffers to disk at some time. It feels somehow more “natural” and safe if we have a copy of the “real” data on disk, and most systems only have limited memory anyway, so with a DB cache capable of holding N blocks, they’ve got to start copying something to disk if they want to change block N+1.

Once we know that we have to copy blocks to disk, we can start asking interesting questions such as “when do we do it?” and “how do we pick which blocks to copy?” These questions take us straight back to lgwr.