Re: autovacuum big table taking hours and sometimes seconds

Thanks, that context is very enlightening. Do you manually vacuum after
doing the big purge of old session data? Is bloat causing issues for you?
Why is it a concern that autovacuum's behavior varies?

*Michael Lewis*

On Thu, Feb 14, 2019 at 12:41 PM Mariel Cherkassky <
mariel(dot)cherkassky(at)gmail(dot)com> wrote:

> Maybe by explaining the tables purpose it will be cleaner. The original
> table contains rows for sessions in my app. Every session saves for itself
> some raw data which is saved in the toasted table. We clean old sessions
> (3+ days) every night. During the day sessions are created so the size of
> the table should grow during the day and freed in the night after the
> autovacuum run.However, the autovacuums sleeps for alot of time and during
> that time more sessions are created so maybe this can explain the big size
> ? Do you think that by increasing the cost limit and decreasing the cost
> delay I can solve the issue ?
>
> On Thu, Feb 14, 2019, 8:38 PM Michael Lewis <mlewis(at)entrata(dot)com wrote:
>
>> It is curious to me that the tuples remaining count varies so wildly. Is
>> this expected?
>>
>>
>> *Michael Lewis*
>>
>> On Thu, Feb 14, 2019 at 9:09 AM Mariel Cherkassky <
>> mariel(dot)cherkassky(at)gmail(dot)com> wrote:
>>
>>> 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
>>>>
>>>