Hello Graham,
I had to deal with "vacuum to prevent wraparound" a few times and it's not fun :(
Postgres doc explains it very well.. if you see that nasty thing running on your db, your best options are:
1. manually vacuum freeze the table
2. examine vacuum costs and lower them to make sure the autovacuum can do it's job. Allow more vacuum processes by tuning autovacuum_max_workers
3. tune and set autovacuum settings for your table to make sure autovacuum picks it up sooner than defaults allow it so.
Depending on your usage, there are some other good optimizations you can try.. The best in my opinion is to partition the large table and isolate old rows from frequently updated rows and manually vacuum freeze rows that you know wan't change. Also newer postgres versions have improved the autovacuuming, if you're on ver 15 (based on your link to docs), you should consider upgrading to more recent version.
Good luck,
-Slava
Can anyone enlighten me on exactly what triggers an "aggressive"
vacuum? I have read the docs multiple times (and watched several
videos!), but the changes I make do not have the expected outcomes; so
I'm clearly missing something.
https://www.postgresql.org/docs/15/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND
I have a fairly large table, that has outgrown the vacuum defaults. We
started by setting:
- autovacuum_vacuum_scale_factor=0
- autovacuum_vacuum_threshold=100000 # 100K
(10% was a lot of tuples). This was definitely an improvement, but
when the big vacuum (freeze) kicked off there was a noticeable
io/latency spike. We then tried setting:
- autovacuum_freeze_table_age=100000000 # 100M
which seemed to make it happen more frequently (and one would hope,
with less work to do). We reduced this to 50M, and again it was more
frequent (but not the 2x I expected). And when I dropped it to 10M,
nothing changed.
After re-reading the docs, I got the impression I also needed to set:
- autovacuum_freeze_min_age=10000000 # 10M
As the system default was 50M. And again, this had an effect, but not
the 5x I was expecting. The docs say:
> all-visible but not all-frozen pages are scanned if the number of transactions that have passed since the last such scan is greater than vacuum_freeze_table_age minus vacuum_freeze_min_age
but wouldn't that have been 10M - 50M? i.e. -40M. Is there some other
setting I'm missing?
All suggestions welcome!
Thanks,
Graham
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