datname | pid | query_start | state | runtime | query
----------------------+-------+---------+------------------+-------------+-----------------+-------------------------------+-----------------------------
postgres | 15088 | 2022-07-01 05:56:22.729101+00 | active | 02:12:56.255367 | SELECT labelfile, spcmapfile, lsn FROM pg_stop_backup(false)
Example Vacuum:
<dbname>=# vacuum verbose analyze pg_database;
INFO: vacuuming "pg_catalog.pg_database"
INFO: "pg_database": found 0 removable, 5 nonremovable row versions in 1 out of 1 pages
DETAIL: 0 dead row versions cannot be removed yet, oldest xmin: 996487475
There were 0 unused item pointers.
Skipped 0 pages due to buffer pins, 0 frozen pages.
0 pages are entirely empty.
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.
INFO: analyzing "pg_catalog.pg_database"
INFO: "pg_database": scanned 1 of 1 pages, containing 5 live rows and 0 dead rows; 5 rows in sample, 5 estimated total rows
VACUUM
Autovacuum from logs of previous issue (timestamp does not match because this is taken from last week logs):
022-06-26 00:00:21 UTC [32632]: [2-1] user=,db=,app=,client= HINT: Close open transactions soon to avoid wraparound problems.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
2022-06-26 00:00:21 UTC [32632]: [3-1] user=,db=,app=,client= LOG: automatic aggressive vacuum of table "postgres.pg_catalog.pg_statistic": index scans: 0
pages: 0 removed, 40 remain, 0 skipped due to pins, 0 skipped frozen
tuples: 0 removed, 1064 remain, 577 are dead but not yet removable, oldest xmin: 4200201112
buffer usage: 57 hits, 43 misses, 0 dirtied
avg read rate: 203.846 MB/s, avg write rate: 0.000 MB/s
system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
2022-06-26 00:00:21 UTC [32632]: [4-1] user=,db=,app=,client= WARNING: oldest xmin is far in the past
2022-06-26 00:00:21 UTC [32632]: [5-1] user=,db=,app=,client= HINT: Close open transactions soon to avoid wraparound problems.
Both xmin point to the long running pg_stop_backup. Not quite sure, but would this cause autovacuum to not kick-in on application tables?
Thanks,
Nikhil
Hi Andrey,./wal-g7 --version
wal-g version v1.1.2-rc 6af461f 2021.12.14_08:19:15 PostgreSQL
The only probable problem I can imagine is that we open transaction during pg_stop_backup() [0] to set a statement_timeoutDo you think this will prevent the autovacuum from kicking in ?I think this tx can be safely removedHow can I remove this? I can test whether this is the problemThanks,NikhilOn Thu, Jun 30, 2022 at 5:18 PM Andrey Borodin <x4mmm@xxxxxxxxxxxxxx> wrote:Hi!
> On 30 Jun 2022, at 14:23, Nikhil Shetty <nikhil.dba04@xxxxxxxxx> wrote:
>
> Hi,
>
> PostgreSQL version - 11.7
>
> We are using wal-g for backup/restore. We had a duration of high WAL generation due to application usage due to which archive backup was delayed.
>
> Backup was started through cron schedule and it didn't complete because it waits for the last WAL to be archived. Now, I know that long running transactions will prevent vacuuming of dead tuples but in this case we were able to run manual vacuums successfully but auto-vacuum never starts.
>
> As soon as we terminated the long running backup session, the auto-vacuum started kicking in.
>
> Wanted to understand why a backup operation is blocking the auto-vacuum?
>
> Wal-g uses non-exclusive backups and waits for the last wal to be archived.
As far as I know, WAL-G should not prevent running autovacuums on tables. WAL-G can be run against replication standby, where vacuum is not possible at all, so there must be no conflicts...
The only probable problem I can imagine is that we open transaction during pg_stop_backup() [0] to set a statement_timeout. I think this is superflous and unneeded. I think this tx can be safely removed. But anyway, this tx does not have a xid (otherwise it could not run on Standby).
BTW which version of WAL-G do you use?
Best regards, Andrey Borodin.
[0] https://github.com/wal-g/wal-g/blob/8b9b3ca06efb44be4724208dc6f6d11836bd34d9/internal/databases/postgres/queryRunner.go#L243
