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Allow users to limit storage reserved by replication slots

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Replication slots are useful to retain data that may be needed by a
replication system.  But experience has shown that allowing them to
retain excessive data can lead to the primary failing because of running
out of space.  This new feature allows the user to configure a maximum
amount of space to be reserved using the new option
max_slot_wal_keep_size.  Slots that overrun that space are invalidated
at checkpoint time, enabling the storage to be released.

Author: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20170228.122736.123383594.horiguchi.kyotaro@lab.ntt.co.jp
This commit is contained in:
Alvaro Herrera
2020-04-07 18:35:00 -04:00
parent b63c293bcb
commit c655077639
17 changed files with 595 additions and 43 deletions
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38
doc/src/sgml/catalogs.sgml
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@ -9907,6 +9907,44 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
</entry>
</row>
<row>
<entry><structfield>wal_status</structfield></entry>
<entry><type>text</type></entry>
<entry></entry>
<entry>Availability of WAL files claimed by this slot.
Possible values are:
<simplelist>
<member>
<literal>normal</literal> means that the claimed files
are within <varname>max_wal_size</varname>
</member>
<member>
<literal>reserved</literal> means that <varname>max_wal_size</varname>
is exceeded but the files are still held, either by some replication
slot or by <varname>wal_keep_segments</varname>
</member>
<member>
<literal>lost</literal> means that some WAL files are definitely lost
and this slot cannot be used to resume replication anymore.
</member>
</simplelist>
The last two states are seen only when
<xref linkend="guc-max-slot-wal-keep-size"/> is
non-negative. If <structfield>restart_lsn</structfield> is NULL, this
field is null.
</entry>
</row>
<row>
<entry><structfield>min_safe_lsn</structfield></entry>
<entry><type>pg_lsn</type></entry>
<entry></entry>
<entry>
The minimum LSN currently available for walsenders.
</entry>
</row>
</tbody>
</tgroup>
</table>

23
doc/src/sgml/config.sgml
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@ -3777,6 +3777,29 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"' # Windows
</listitem>
</varlistentry>
<varlistentry id="guc-max-slot-wal-keep-size" xreflabel="max_slot_wal_keep_size">
<term><varname>max_slot_wal_keep_size</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_slot_wal_keep_size</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<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 -1 (the default),
replication slots retain unlimited amount of WAL files. If
restart_lsn of a replication slot gets behind more than that megabytes
from the current LSN, the standby using the slot may no longer be able
to continue replication due to removal of required WAL files. You
can see the WAL availability of replication slots
in <link linkend="view-pg-replication-slots">pg_replication_slots</link>.
</para>
</listitem>
</varlistentry>
<varlistentry id="guc-wal-sender-timeout" xreflabel="wal_sender_timeout">
<term><varname>wal_sender_timeout</varname> (<type>integer</type>)
<indexterm>

8
doc/src/sgml/high-availability.sgml
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@ -925,9 +925,11 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
<xref linkend="guc-archive-command"/>.
However, these methods often result in retaining more WAL segments than
required, whereas replication slots retain only the number of segments
known to be needed. An advantage of these methods is that they bound
the space requirement for <literal>pg_wal</literal>; there is currently no way
to do this using replication slots.
known to be needed. On the other hand, replication slots can retain so
many WAL segments that they fill up the space allocated
for <literal>pg_wal</literal>;
<xref linkend="guc-max-slot-wal-keep-size"/> limits the size of WAL files
retained by replication slots.
</para>
<para>
Similarly, <xref linkend="guc-hot-standby-feedback"/>