Tom Lane wrote:
Anyway, if there are no XLOG records since the last checkpoint, there's
probably nothing in shared buffers that needs flushing.  There might be
some dirty hint-bits, but the only reason to push those out is to make
some free buffers available, and doing that is not checkpoint's job (nor
the all-buffers scan's job); that's what the LRU scan is for.

Yeah, except the LRU scan is not doing a very good job at that. It will ignore buffers with usage_count > 0, and it only scans bgwriter_lru_percent buffers ahead of the clock hand.

One pathological case is a COPY of a table slightly smaller than shared_buffers. That will fill the buffer cache. If you then have a checkpoint, and after that a SELECT COUNT(*), or a VACUUM, the buffer cache will be full of pages with just hint-bit-updates, but no WAL activity since last checkpoint.

But let's fix the LRU scan, rather work around it's deficiencies.

  Heikki Linnakangas

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