Hello. At Thu, 6 Sep 2018 22:32:21 +0200, Peter Eisentraut <peter.eisentr...@2ndquadrant.com> wrote in <29bbd79d-696b-509e-578a-0fc38a3b9...@2ndquadrant.com> > This documentation > > + <para> > + Specify the maximum size of WAL files > + that <link linkend="streaming-replication-slots">replication > + slots</link> are allowed to retain in the > <filename>pg_wal</filename> > + directory at checkpoint time. > + If <varname>max_slot_wal_keep_size</varname> is zero (the default), > + replication slots retain unlimited size of WAL files. > + </para> > > doesn't say anything about what happens when the limit is exceeded. > Does the system halt until the WAL is fetched from the slots? Do the > slots get invalidated?
Thanks for pointing that. That's a major cause of confusion. Does the following make sense? > Specify the maximum size of WAL files that <link > linkend="streaming-replication-slots">replication slots</link> > are allowed to retain in the <filename>pg_wal</filename> > directory at checkpoint time. If > <varname>max_slot_wal_keep_size</varname> is zero (the > default), replication slots retain unlimited size of WAL files. + If restart_lsn of a replication slot gets behind more than that + bytes from the current LSN, the standby using the slot may not + be able to reconnect due to removal of required WAL records. And the following sentense is wrong now. I'll remove it in the coming version 9. > <para> > This parameter is used being rounded down to the multiples of WAL file > size. > </para> > Also, I don't think 0 is a good value for the default behavior. 0 would > mean that a slot is not allowed to retain any more WAL than already > exists anyway. Maybe we don't want to support that directly, but it's a > valid configuration. So maybe use -1 for infinity. In realtion to the reply just sent to Sawada-san, remain of a slot can be at most 16MB in the 0 case with the default segment size. So you're right in this sense. Will fix in the coming version. Thanks. =# show max_slot_wal_keep_size; max_slot_wal_keep_size ------------------------ 0 (1 row) =# select pg_current_wal_lsn(), restart_lsn, remain, pg_size_pretty(remain) as remain from pg_replication_slots ; pg_current_wal_lsn | restart_lsn | remain | remain --------------------+-------------+----------+-------- 0/4000000 | 0/4000000 | 16777216 | 16 MB (1 row) .... =# select pg_current_wal_lsn(), restart_lsn, remain, pg_size_pretty(remain) as remain from pg_replication_slots ; pg_current_wal_lsn | restart_lsn | remain | remain --------------------+-------------+--------+-------- 0/4FF46D8 | 0/4FF46D8 | 47400 | 46 kB (1 row) regards. -- Kyotaro Horiguchi NTT Open Source Software Center
