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Commit4830f10

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Disable vacuum's use of a buffer access strategy during failsafe
Traditionally, vacuum always makes use of a buffer access strategy 32buffers in size. This means that running vacuums tend not to cause toomany shared buffers to become dirty, however, this can cause vacuums torun much more slowly than they otherwise could as WAL flushes will occurmore frequently due to having to flush WAL out to the LSN of the dirtypage before that page can be written to disk.When we are performing failsafe VACUUMs (as added in1e55e7d), we reallywant to make the vacuum work go as quickly as possible, so here we disablethe buffer access strategy when entering failsafe mode while vacuuming arelation.Per idea and analyis from Andres Freund.In passing, also include some changes I had intended for32fbe02.Author: Melanie PlagemanReviewed-by: Justin Pryzby, David RowleyDiscussion:https://postgr.es/m/20230111182720.ejifsclfwymw2reb%40awork3.anarazel.de
1 parent525fb0a commit4830f10

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3 files changed

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3 files changed

+18
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‎doc/src/sgml/config.sgml

Lines changed: 6 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -9320,8 +9320,12 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
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</para>
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<para>
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When the failsafe is triggered, any cost-based delay that is
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in effect will no longer be applied, and further non-essential
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maintenance tasks (such as index vacuuming) are bypassed.
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in effect will no longer be applied, further non-essential
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maintenance tasks (such as index vacuuming) are bypassed, and any
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<glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
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in use will be disabled resulting in <command>VACUUM</command> being
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free to make use of all of
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<glossterm linkend="glossary-shared-memory">shared buffers</glossterm>.
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</para>
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<para>
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The default is 1.6 billion transactions. Although users can

‎src/backend/access/heap/vacuumlazy.c

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Original file line numberDiff line numberDiff line change
@@ -2623,6 +2623,13 @@ lazy_check_wraparound_failsafe(LVRelState *vacrel)
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{
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vacrel->failsafe_active= true;
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/*
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* Abandon use of a buffer access strategy to allow use of all of
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* shared buffers. We assume the caller who allocated the memory for
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* the BufferAccessStrategy will free it.
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*/
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vacrel->bstrategy=NULL;
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/* Disable index vacuuming, index cleanup, and heap rel truncation */
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vacrel->do_index_vacuuming= false;
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vacrel->do_index_cleanup= false;

‎src/backend/commands/vacuum.c

Lines changed: 5 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -391,11 +391,13 @@ vacuum(List *relations, VacuumParams *params,
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/*
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* If caller didn't give us a buffer strategy object, make one in the
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* cross-transaction memory context.
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* cross-transaction memory context. We needn't bother making this for
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* VACUUM (FULL) or VACUUM (ONLY_DATABASE_STATS) as they'll not make use
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* of it.
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*/
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if (bstrategy==NULL&&
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!(params->options&VACOPT_ONLY_DATABASE_STATS||
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params->options&VACOPT_FULL))
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(params->options&(VACOPT_ONLY_DATABASE_STATS |
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VACOPT_FULL))==0)
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{
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MemoryContextold_context=MemoryContextSwitchTo(vac_context);
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