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Adjust max_slot_wal_keep_size behavior per review

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In pg_replication_slot, change output from normal/reserved/lost to
reserved/extended/unreserved/ lost, which better expresses the possible
states particularly near the time where segments are no longer safe but
checkpoint has not run yet.

Under the new definition, reserved means the slot is consuming WAL
that's still under the normal WAL size constraints; extended means it's
consuming WAL that's being protected by wal_keep_segments or the slot
itself, whose size is below max_slot_wal_keep_size; unreserved means the
WAL is no longer safe, but checkpoint has not yet removed those files.
Such as slot is in imminent danger, but can still continue for a little
while and may catch up to the reserved WAL space.

Also, there were some bugs in the calculations used to report the
status; fixed those.

Backpatch to 13.

Reported-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200616.120236.1809496990963386593.horikyota.ntt@gmail.com
This commit is contained in:
Alvaro Herrera
2020-06-24 14:23:39 -04:00
parent 12e52ba5a7
commit 6f7a862bed
5 changed files with 97 additions and 54 deletions
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24
doc/src/sgml/catalogs.sgml
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@ -11239,19 +11239,29 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
Possible values are:
<itemizedlist>
<listitem>
<para><literal>normal</literal> means that the claimed files
<para><literal>reserved</literal> means that the claimed files
are within <varname>max_wal_size</varname>.</para>
</listitem>
<listitem>
<para><literal>reserved</literal> means
<para><literal>extended</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>.</para>
still retained, either by the replication slot or
by <varname>wal_keep_segments</varname>.
</para>
</listitem>
<listitem>
<para><literal>lost</literal> means that some WAL files are
definitely lost and this slot cannot be used to resume replication
anymore.</para>
<para>
<literal>unreserved</literal> means that the slot no longer
retains the required WAL files and some of them are to be removed at
the next checkpoint. This state can return
to <literal>reserved</literal> or <literal>extended</literal>.
</para>
</listitem>
<listitem>
<para>
<literal>lost</literal> means that some required WAL files have
been removed and this slot is no longer usable.
</para>
</listitem>
</itemizedlist>
The last two states are seen only when