for 15 years pgfincore has been sitting quietly and being used in large setups to help in HA and resources management.
It can perfectly stay as is, to be honest I was expecting to one day include a windows support and propose that to PostgreSQL, it appears getting support on linux and BSD is more than enough today.So I wonder if there are interest for having virtual memory snapshot and restore operations with, for example, pg_prewarm/autowarm ?
IMHO, to improve the user experience here we'd need something that combined the abilities of all these extensions into a cohesive interface that allowed users to simply say "please get this data into cache". Simply moving pgfincore into core Postgres wouldn't satisfy that need.
So I think the real question is whether the community feels spport for better cache (buffercache + filesystem) management is a worthwhile feature to add to Postgres.
Micromanaging cache contents for periodic jobs seems almost like a mis-feature. While it's a useful tool to have in the toolbox, it's also a non-trivial layer of complexity. IMHO not something we'd want to add. Though, there might be smaller items that would make creating tools to do that easier, such as some ability to see what blocks a backend is accessing (perhaps via a hook).
On the surface, improving RTO via cache warming sounds
interesting ... but I'm not sure how useful it would be in
reality. Users that care about RTO would almost always have some
form of hot-standby, and generally those will already have a lot
of data in cache. While they won't have read-only data in cache, I
have to wonder if the answer to that problem is allowing writers
to tell a replica what blocks are being read, so the replica can
keep them in cache. Also, most (all?) operations that require a
restart could be handled via a failover, so I'm not sure how much
cache management moves the needle there.
Some usecases covered: snapshot/restore cache around cronjobs, around dumps, switchover, failover, on stop/start of postgres (think kernel upgrade with a cold restart), ...
pgfincore also provides some nice information with mincore (on FreeBSD mincore is more interesting) or cachestat, again it can remain as an out of tree extension but I will be happy to add to commitfest if there are interest from the community.
An example of cachestat output:postgres=# select *from vm_relation_cachestat('foo',range:=1024*32);
block_start | block_count | nr_pages | nr_cache | nr_dirty | nr_writeback | nr_evicted | nr_recently_evicted
-------------+-------------+----------+----------+----------+--------------+------------+---------------------
0 | 32768 | 65536 | 62294 | 0 | 0 | 3242 | 3242
32768 | 32768 | 65536 | 39279 | 0 | 0 | 26257 | 26257
65536 | 32768 | 65536 | 22516 | 0 | 0 | 43020 | 43020
98304 | 32768 | 65536 | 24944 | 0 | 0 | 40592 | 40592
131072 | 1672 | 3344 | 487 | 0 | 0 | 2857 | 2857
Comments?
--- Cédric Villemain +33 (0)6 20 30 22 52 https://Data-Bene.io PostgreSQL Expertise, Support, Training, R&D
-- Jim Nasby, Data Architect, Austin TX