Postgresql源码（11）StrategyGetBuffer新buffer分配流程图

Background Writer's Processing
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The background writer is designed to write out pages that are likely to be
recycled soon, thereby offloading the writing work from active backends.
To do this, it scans forward circularly from the current position of
nextVictimBuffer (which it does not change!), looking for buffers that are
dirty and not pinned nor marked with a positive usage count.  It pins,
writes, and releases any such buffer.

If we can assume that reading nextVictimBuffer is an atomic action, then
the writer doesn't even need to take buffer_strategy_lock in order to look
for buffers to write; it needs only to spinlock each buffer header for long
enough to check the dirtybit.  Even without that assumption, the writer
only needs to take the lock long enough to read the variable value, not
while scanning the buffers.  (This is a very substantial improvement in
the contention cost of the writer compared to PG 8.0.)

The background writer takes shared content lock on a buffer while writing it
out (and anyone else who flushes buffer contents to disk must do so too).
This ensures that the page image transferred to disk is reasonably consistent.
We might miss a hint-bit update or two but that isn't a problem, for the same
reasons mentioned under buffer access rules.

As of 8.4, background writer starts during recovery mode when there is
some form of potentially extended recovery to perform. It performs an
identical service to normal processing, except that checkpoints it
writes are technically restartpoints.