Re: autovacuum big table taking hours and sometimes seconds

I checked in the logs when the autovacuum vacuum my big toasted table
during the week and I wanted to confirm with you what I think :
postgresql-Fri.log:2019-02-08 05:05:53 EST 24776 LOG: automatic vacuum
of table "myDB.pg_toast.pg_toast_1958391": index scans: 8
postgresql-Fri.log- pages: 2253 removed, 13737828 remain
postgresql-Fri.log- tuples: 21759258 removed, 27324090 remain
postgresql-Fri.log- buffer usage: 15031267 hits, 21081633 misses, 19274530
dirtied
postgresql-Fri.log- avg read rate: 2.700 MiB/s, avg write rate: 2.469 MiB/s
--
postgresql-Mon.log:2019-02-11 01:11:46 EST 8426 LOG: automatic vacuum of
table "myDB.pg_toast.pg_toast_1958391": index scans: 23
postgresql-Mon.log- pages: 0 removed, 23176876 remain
postgresql-Mon.log- tuples: 62269200 removed, 82958 remain
postgresql-Mon.log- buffer usage: 28290538 hits, 46323736 misses, 38950869
dirtied
postgresql-Mon.log- avg read rate: 2.850 MiB/s, avg write rate: 2.396 MiB/s
--
postgresql-Mon.log:2019-02-11 21:43:19 EST 24323 LOG: automatic vacuum
of table "myDB.pg_toast.pg_toast_1958391": index scans: 1
postgresql-Mon.log- pages: 0 removed, 23176876 remain
postgresql-Mon.log- tuples: 114573 removed, 57785 remain
postgresql-Mon.log- buffer usage: 15877931 hits, 15972119 misses, 15626466
dirtied
postgresql-Mon.log- avg read rate: 2.525 MiB/s, avg write rate: 2.470 MiB/s
--
postgresql-Sat.log:2019-02-09 04:54:50 EST 1793 LOG: automatic vacuum of
table "myDB.pg_toast.pg_toast_1958391": index scans: 13
postgresql-Sat.log- pages: 0 removed, 13737828 remain
postgresql-Sat.log- tuples: 34457593 removed, 15871942 remain
postgresql-Sat.log- buffer usage: 15552642 hits, 26130334 misses, 22473776
dirtied
postgresql-Sat.log- avg read rate: 2.802 MiB/s, avg write rate: 2.410 MiB/s
--
postgresql-Thu.log:2019-02-07 12:08:50 EST 29630 LOG: automatic vacuum
of table "myDB.pg_toast.pg_toast_1958391": index scans: 13
postgresql-Thu.log- pages: 0 removed, 10290976 remain
postgresql-Thu.log- tuples: 35357057 removed, 3436237 remain
postgresql-Thu.log- buffer usage: 11854053 hits, 21346342 misses, 19232835
dirtied
postgresql-Thu.log- avg read rate: 2.705 MiB/s, avg write rate: 2.437 MiB/s
--
postgresql-Tue.log:2019-02-12 20:54:44 EST 21464 LOG: automatic vacuum
of table "myDB.pg_toast.pg_toast_1958391": index scans: 10
postgresql-Tue.log- pages: 0 removed, 23176876 remain
postgresql-Tue.log- tuples: 26011446 removed, 49426774 remain
postgresql-Tue.log- buffer usage: 21863057 hits, 28668178 misses, 25472137
dirtied
postgresql-Tue.log- avg read rate: 2.684 MiB/s, avg write rate: 2.385 MiB/s
--

Lets focus for example on one of the outputs :
postgresql-Fri.log:2019-02-08 05:05:53 EST 24776 LOG: automatic vacuum
of table "myDB.pg_toast.pg_toast_1958391": index scans: 8
postgresql-Fri.log- pages: 2253 removed, 13737828 remain
postgresql-Fri.log- tuples: 21759258 removed, 27324090 remain
postgresql-Fri.log- buffer usage: *15031267* hits, *21081633 *misses, *19274530
*dirtied
postgresql-Fri.log- avg read rate: 2.700 MiB/s, avg write rate: 2.469 MiB/s

The cost_limit is set to 200 (default) and the cost_delay is set to 20ms.
The calculation I did : (1**15031267*+10**21081633*+20**19274530)*/200*20/1000
= 61133.8197 seconds ~ 17H
So autovacuum was laying down for 17h ? I think that I should increase the
cost_limit to max specifically on the toasted table. What do you think ? Am
I wrong here ?

‫בתאריך יום ה׳, 7 בפבר׳ 2019 ב-18:26 מאת ‪Jeff Janes‬‏ <‪
jeff(dot)janes(at)gmail(dot)com‬‏>:‬

> On Thu, Feb 7, 2019 at 6:55 AM Mariel Cherkassky <
> mariel(dot)cherkassky(at)gmail(dot)com> wrote:
>
> I have 3 questions :
>> 1)To what value do you recommend to increase the vacuum cost_limit ? 2000
>> seems reasonable ? Or maybe its better to leave it as default and assign a
>> specific value for big tables ?
>>
>
> That depends on your IO hardware, and your workload. You wouldn't want
> background vacuum to use so much of your available IO that it starves your
> other processes.
>
>
>
>> 2)When the autovacuum reaches the cost_limit while trying to vacuum a
>> specific table, it wait nap_time seconds and then it continue to work on
>> the same table ?
>>
>
> No, it waits for autovacuum_vacuum_cost_delay before resuming within the
> same table. During this delay, the table is still open and it still holds a
> lock on it, and holds the transaction open, etc. Naptime is entirely
> different, it controls how often the vacuum scheduler checks to see which
> tables need to be vacuumed again.
>
>
>
>> 3)So in case I have a table that keeps growing (not fast because I set
>> the vacuum_scale_factor to 0 and the autovacuum_vacuum_threshold to 10000).
>> If the table keep growing it means I should try to increase the cost right
>> ? Do you see any other option ?
>>
>
> You can use pg_freespacemap to see if the free space is spread evenly
> throughout the table, or clustered together. That might help figure out
> what is going on. And, is it the table itself that is growing, or the
> index on it?
>
> Cheers,
>
> Jeff
>