Re: [BUGS] BUG #14538: streaming replication same wal missing

On Thu, Feb 9, 2017 at 10:58 PM,  <vodevsh@gmail.com> wrote:
> Bug reference:      14538
> Logged by:          Vladimir Svedov
> Email address:      vodevsh@gmail.com
> PostgreSQL version: 9.3.11

You should really update to 9.3.16, the latest minor release in the
9.3 series. You are missing more than 1 year worth of bug fixes here.

> Several days ago I saw this in master log:
>
>  replica 2017-02-06 16:10:55 UTC ERROR:  requested WAL segment
> 000000030000096D00000052 has already been removed
> This message was repeating until I stopped slave.

That happens :"(

> Checking pg_stat_activity on prod showed autovacuum task on table ~250GB, I
> decided that it produced huge amount of WALs. Checking them confirmed my
> assumption - all wal_keep_segments=500 were last minute. So I thought that
> network bandwidth was not sufficient to send WALs fast enough to slave to
> replay them. Master has archive_command = cd ., so no way back.
>
> I did not panic, I recreate replication. I:
>
> delete all data files on slave
> pg_start_backup() on prod
> rsync -zahv data
> pg_start slave

Does this line mean that you are starting the slave? You may want to
start it after issuing pg_stop_backup() so a segment switch is done
and a standby is able to fetch all the data it needs has been properly
archived...

> pg_stop_backup() on prod and check logs.
> Imagine my desperation to see that slave has this:
> -bash-4.2$ head -n 20 /pg/data93/pg_log/postgresql-2017-02-09_121514.log
> LOG:  could not open usermap file "/pg/data93/pg_ident.conf": No such file
> or directory
> LOG:  database system was shut down in recovery at 2017-02-09 12:15:07 UTC
> LOG:  entering standby mode
> LOG:  redo starts at 982/39074640
> FATAL:  the database system is starting up
> FATAL:  the database system is starting up
> FATAL:  the database system is starting up
> LOG:  consistent recovery state reached at 982/3AEC60F8
> LOG:  unexpected pageaddr 97C/83EC8000 in log segment
> 00000003000009820000003A, offset 15499264
> LOG:  database system is ready to accept read only connections
> LOG:  started streaming WAL from primary at 982/3A000000 on timeline 3
> ERROR:  requested WAL segment 000000030000096D00000052 has already been
> removed

This error message is being generated by a WAL sender running on your
new *slave*, meaning that there is a cascading standby connected to
this slave server and trying to fetch this WAL segment. But it does
not have it as it has been recycled already during recovery.

If the slave you just created was complaining about a missing segment
from its primary it would throw that instead:
FATAL:  could not receive data from WAL stream: ERROR:  requested WAL
segment 000000010000000000000007 has already been removed

In short your new slave is fine, but you need to rebuild the cascading
standby behind though.
-- 
Michael


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