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.cherkas...@gmail.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 <mle...@entrata.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.cherkas...@gmail.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.ja...@gmail.com>: >>> >>>> On Thu, Feb 7, 2019 at 6:55 AM Mariel Cherkassky < >>>> mariel.cherkas...@gmail.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 >>>> >>>
