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Prevent concurrent SimpleLruTruncate() for any given SLRU.

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The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com
This commit is contained in:
Noah Misch
2020-08-15 10:15:53 -07:00
parent dea07098af
commit e525770dd5
12 changed files with 119 additions and 16 deletions
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4
doc/src/sgml/catalogs.sgml
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@ -8886,7 +8886,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</>:<replaceable>&lt;salt&gt;<
and general database objects (identified by class OID and object OID,
in the same way as in <structname>pg_description</structname> or
<structname>pg_depend</structname>). Also, the right to extend a
relation is represented as a separate lockable object.
relation is represented as a separate lockable object, as is the right to
update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>.
Also, <quote>advisory</> locks can be taken on numbers that have
user-defined meanings.
</para>
@ -8912,6 +8913,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</>:<replaceable>&lt;salt&gt;<
Type of the lockable object:
<literal>relation</>,
<literal>extend</>,
<literal>frozenid</literal>,
<literal>page</>,
<literal>tuple</>,
<literal>transactionid</>,

20
doc/src/sgml/monitoring.sgml
View File

@ -845,7 +845,7 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<tbody>
<row>
<entry morerows="62"><literal>LWLock</></entry>
<entry morerows="64"><literal>LWLock</></entry>
<entry><literal>ShmemIndexLock</></entry>
<entry>Waiting to find or allocate space in shared memory.</entry>
</row>
@ -1043,6 +1043,16 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry>Waiting to execute <function>txid_status</function> or update
the oldest transaction id available to it.</entry>
</row>
<row>
<entry><literal>WrapLimitsVacuumLock</literal></entry>
<entry>Waiting to update limits on transaction id and multixact
consumption.</entry>
</row>
<row>
<entry><literal>NotifyQueueTailLock</literal></entry>
<entry>Waiting to update limit on notification message
storage.</entry>
</row>
<row>
<entry><literal>clog</></entry>
<entry>Waiting for I/O on a clog (transaction status) buffer.</entry>
@ -1118,7 +1128,7 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry>Waiting for TBM shared iterator lock.</entry>
</row>
<row>
<entry morerows="9"><literal>Lock</></entry>
<entry morerows="10"><literal>Lock</></entry>
<entry><literal>relation</></entry>
<entry>Waiting to acquire a lock on a relation.</entry>
</row>
@ -1126,6 +1136,12 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry><literal>extend</></entry>
<entry>Waiting to extend a relation.</entry>
</row>
<row>
<entry><literal>frozenid</literal></entry>
<entry>Waiting to
update <structname>pg_database</structname>.<structfield>datfrozenxid</structfield>
and <structname>pg_database</structname>.<structfield>datminmxid</structfield>.</entry>
</row>
<row>
<entry><literal>page</></entry>
<entry>Waiting to acquire a lock on page of a relation.</entry>