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Skip WAL for new relfilenodes, under wal_level=minimal.

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Until now, only selected bulk operations (e.g. COPY) did this.  If a
given relfilenode received both a WAL-skipping COPY and a WAL-logged
operation (e.g. INSERT), recovery could lose tuples from the COPY.  See
src/backend/access/transam/README section "Skipping WAL for New
RelFileNode" for the new coding rules.  Maintainers of table access
methods should examine that section.

To maintain data durability, just before commit, we choose between an
fsync of the relfilenode and copying its contents to WAL.  A new GUC,
wal_skip_threshold, guides that choice.  If this change slows a workload
that creates small, permanent relfilenodes under wal_level=minimal, try
adjusting wal_skip_threshold.  Users setting a timeout on COMMIT may
need to adjust that timeout, and log_min_duration_statement analysis
will reflect time consumption moving to COMMIT from commands like COPY.

Internally, this requires a reliable determination of whether
RollbackAndReleaseCurrentSubTransaction() would unlink a relation's
current relfilenode.  Introduce rd_firstRelfilenodeSubid.  Amend the
specification of rd_createSubid such that the field is zero when a new
rel has an old rd_node.  Make relcache.c retain entries for certain
dropped relations until end of transaction.

Bump XLOG_PAGE_MAGIC, since this introduces XLOG_GIST_ASSIGN_LSN.
Future servers accept older WAL, so this bump is discretionary.

Kyotaro Horiguchi, reviewed (in earlier, similar versions) by Robert
Haas.  Heikki Linnakangas and Michael Paquier implemented earlier
designs that materially clarified the problem.  Reviewed, in earlier
designs, by Andrew Dunstan, Andres Freund, Alvaro Herrera, Tom Lane,
Fujii Masao, and Simon Riggs.  Reported by Martijn van Oosterhout.

Discussion: https://postgr.es/m/20150702220524.GA9392@svana.org
This commit is contained in:
Noah Misch
2020-04-04 12:25:34 -07:00
parent 552fcebff0
commit c6b92041d3
53 changed files with 1566 additions and 372 deletions
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58
src/backend/commands/copy.c
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@ -2714,63 +2714,15 @@ CopyFrom(CopyState cstate)
RelationGetRelationName(cstate->rel))));
}
/*----------
* Check to see if we can avoid writing WAL
*
* If archive logging/streaming is not enabled *and* either
* - table was created in same transaction as this COPY
* - data is being written to relfilenode created in this transaction
* then we can skip writing WAL. It's safe because if the transaction
* doesn't commit, we'll discard the table (or the new relfilenode file).
* If it does commit, we'll have done the table_finish_bulk_insert() at
* the bottom of this routine first.
*
* As mentioned in comments in utils/rel.h, the in-same-transaction test
* is not always set correctly, since in rare cases rd_newRelfilenodeSubid
* can be cleared before the end of the transaction. The exact case is
* when a relation sets a new relfilenode twice in same transaction, yet
* the second one fails in an aborted subtransaction, e.g.
*
* BEGIN;
* TRUNCATE t;
* SAVEPOINT save;
* TRUNCATE t;
* ROLLBACK TO save;
* COPY ...
*
* Also, if the target file is new-in-transaction, we assume that checking
* FSM for free space is a waste of time, even if we must use WAL because
* of archiving. This could possibly be wrong, but it's unlikely.
*
* The comments for table_tuple_insert and RelationGetBufferForTuple
* specify that skipping WAL logging is only safe if we ensure that our
* tuples do not go into pages containing tuples from any other
* transactions --- but this must be the case if we have a new table or
* new relfilenode, so we need no additional work to enforce that.
*
* We currently don't support this optimization if the COPY target is a
* partitioned table as we currently only lazily initialize partition
* information when routing the first tuple to the partition. We cannot
* know at this stage if we can perform this optimization. It should be
* possible to improve on this, but it does mean maintaining heap insert
* option flags per partition and setting them when we first open the
* partition.
*
* This optimization is not supported for relation types which do not
* have any physical storage, with foreign tables and views using
* INSTEAD OF triggers entering in this category. Partitioned tables
* are not supported as per the description above.
*----------
/*
* If the target file is new-in-transaction, we assume that checking FSM
* for free space is a waste of time. This could possibly be wrong, but
* it's unlikely.
*/
/* createSubid is creation check, newRelfilenodeSubid is truncation check */
if (RELKIND_HAS_STORAGE(cstate->rel->rd_rel->relkind) &&
(cstate->rel->rd_createSubid != InvalidSubTransactionId ||
cstate->rel->rd_newRelfilenodeSubid != InvalidSubTransactionId))
{
cstate->rel->rd_firstRelfilenodeSubid != InvalidSubTransactionId))
ti_options |= TABLE_INSERT_SKIP_FSM;
if (!XLogIsNeeded())
ti_options |= TABLE_INSERT_SKIP_WAL;
}
/*
* Optimize if new relfilenode was created in this subxact or one of its