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 ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
