Thread: Aggressive vacuum
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 scanis 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
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
On Thu, Dec 19, 2024 at 5:37 AM Graham Hay <grahamrhay@gmail.com> wrote:
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
--
-slava
On Thu, Dec 19, 2024 at 8:37 AM Graham Hay <grahamrhay@gmail.com> wrote:
https://www.postgresql.org/docs/15/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND
This link is to v15 docs. Is that the version you are actually using? The behavior of this feature has changed (repeatedly) over the years, so this is important to know.
This was definitely an improvement, but
when the big vacuum (freeze) kicked off there was a noticeable
io/latency spike.
Was the change noticeable because it caused degradation in the user experience (latency of things users actually care about) or because you have monitoring tools which detected a change even though no one was complaining? Can you tell if the cause is driven by sequential reads, random reads, or writes?
The way to suppress IO spikes is generally to tweak the vacuum_cost_* and/or auto_vacuum_vacuum_cost_* settings. This will make the vacuums take longer but be less intensive.
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.
Please be more quantitative. It wasn't 2x, but then what was it? And how long did each one take?
Freezing automatically more often is likely not the answer to IO spikes. Going from gumming up your system once a week for one hour to gumming it up 3 times a week for 20 minutes each is likely not a real solution. And it might not even do that--you could gum it up 3 times a week for 50 minutes each! Unless you have large swaths of table which become effectively read-only over time, each aggressive vacuum might need to do the same amount of IO so doing it more often just makes things worse.
The solution would be to either use the *vacuum_cost_* parameters to throttle it down to the point where it doesn't cause problems, or intentionally schedule vacuum freeze during quiet periods (over night, over weekends) preempting the need for them to happen automatically.
Cheers,
Jeff