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
EnterpriseDB http://www.enterprisedb.com
