Re: autovacuum big table taking hours and sometimes seconds

No I don't run vacuum manually afterwards because the autovacuum should
run. This process happens every night. Yes , bloating is an issue because
the table grow and take a lot of space on disk. Regarding the autovacuum,
I think that it sleeps too much time (17h) during it's work, don't you
think so?

On Thu, Feb 14, 2019, 9:52 PM Michael Lewis <mlewis(at)entrata(dot)com wrote:

> 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
>>>>>
>>>>