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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >WAL Configuration</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Reliability and the Write-Ahead Log" HREF="wal.html"><LINK REL="PREVIOUS" TITLE="Asynchronous Commit" HREF="wal-async-commit.html"><LINK REL="NEXT" TITLE="WAL Internals" HREF="wal-internals.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Asynchronous Commit" HREF="wal-async-commit.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="wal.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 29. Reliability and the Write-Ahead Log</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="WAL Internals" HREF="wal-internals.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="WAL-CONFIGURATION" >29.4. <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > Configuration</A ></H1 ><P > There are several <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM >-related configuration parameters that affect database performance. This section explains their use. Consult <A HREF="runtime-config.html" >Chapter 18</A > for general information about setting server configuration parameters. </P ><P > <I CLASS="FIRSTTERM" >Checkpoints</I > are points in the sequence of transactions at which it is guaranteed that the heap and index data files have been updated with all information written before the checkpoint. At checkpoint time, all dirty data pages are flushed to disk and a special checkpoint record is written to the log file. (The changes were previously flushed to the <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > files.) In the event of a crash, the crash recovery procedure looks at the latest checkpoint record to determine the point in the log (known as the redo record) from which it should start the REDO operation. Any changes made to data files before that point are guaranteed to be already on disk. Hence, after a checkpoint, log segments preceding the one containing the redo record are no longer needed and can be recycled or removed. (When <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > archiving is being done, the log segments must be archived before being recycled or removed.) </P ><P > The checkpoint requirement of flushing all dirty data pages to disk can cause a significant I/O load. For this reason, checkpoint activity is throttled so I/O begins at checkpoint start and completes before the next checkpoint starts; this minimizes performance degradation during checkpoints. </P ><P > The server's checkpointer process automatically performs a checkpoint every so often. A checkpoint is created every <A HREF="runtime-config-wal.html#GUC-CHECKPOINT-SEGMENTS" >checkpoint_segments</A > log segments, or every <A HREF="runtime-config-wal.html#GUC-CHECKPOINT-TIMEOUT" >checkpoint_timeout</A > seconds, whichever comes first. The default settings are 3 segments and 300 seconds (5 minutes), respectively. In cases where no WAL has been written since the previous checkpoint, new checkpoints will be skipped even if checkpoint_timeout has passed. If WAL archiving is being used and you want to put a lower limit on how often files are archived in order to bound potential data loss, you should adjust archive_timeout parameter rather than the checkpoint parameters. It is also possible to force a checkpoint by using the SQL command <TT CLASS="COMMAND" >CHECKPOINT</TT >. </P ><P > Reducing <TT CLASS="VARNAME" >checkpoint_segments</TT > and/or <TT CLASS="VARNAME" >checkpoint_timeout</TT > causes checkpoints to occur more often. This allows faster after-crash recovery (since less work will need to be redone). However, one must balance this against the increased cost of flushing dirty data pages more often. If <A HREF="runtime-config-wal.html#GUC-FULL-PAGE-WRITES" >full_page_writes</A > is set (as is the default), there is another factor to consider. To ensure data page consistency, the first modification of a data page after each checkpoint results in logging the entire page content. In that case, a smaller checkpoint interval increases the volume of output to the WAL log, partially negating the goal of using a smaller interval, and in any case causing more disk I/O. </P ><P > Checkpoints are fairly expensive, first because they require writing out all currently dirty buffers, and second because they result in extra subsequent WAL traffic as discussed above. It is therefore wise to set the checkpointing parameters high enough that checkpoints don't happen too often. As a simple sanity check on your checkpointing parameters, you can set the <A HREF="runtime-config-wal.html#GUC-CHECKPOINT-WARNING" >checkpoint_warning</A > parameter. If checkpoints happen closer together than <TT CLASS="VARNAME" >checkpoint_warning</TT > seconds, a message will be output to the server log recommending increasing <TT CLASS="VARNAME" >checkpoint_segments</TT >. Occasional appearance of such a message is not cause for alarm, but if it appears often then the checkpoint control parameters should be increased. Bulk operations such as large <TT CLASS="COMMAND" >COPY</TT > transfers might cause a number of such warnings to appear if you have not set <TT CLASS="VARNAME" >checkpoint_segments</TT > high enough. </P ><P > To avoid flooding the I/O system with a burst of page writes, writing dirty buffers during a checkpoint is spread over a period of time. That period is controlled by <A HREF="runtime-config-wal.html#GUC-CHECKPOINT-COMPLETION-TARGET" >checkpoint_completion_target</A >, which is given as a fraction of the checkpoint interval. The I/O rate is adjusted so that the checkpoint finishes when the given fraction of <TT CLASS="VARNAME" >checkpoint_segments</TT > WAL segments have been consumed since checkpoint start, or the given fraction of <TT CLASS="VARNAME" >checkpoint_timeout</TT > seconds have elapsed, whichever is sooner. With the default value of 0.5, <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > can be expected to complete each checkpoint in about half the time before the next checkpoint starts. On a system that's very close to maximum I/O throughput during normal operation, you might want to increase <TT CLASS="VARNAME" >checkpoint_completion_target</TT > to reduce the I/O load from checkpoints. The disadvantage of this is that prolonging checkpoints affects recovery time, because more WAL segments will need to be kept around for possible use in recovery. Although <TT CLASS="VARNAME" >checkpoint_completion_target</TT > can be set as high as 1.0, it is best to keep it less than that (perhaps 0.9 at most) since checkpoints include some other activities besides writing dirty buffers. A setting of 1.0 is quite likely to result in checkpoints not being completed on time, which would result in performance loss due to unexpected variation in the number of WAL segments needed. </P ><P > There will always be at least one WAL segment file, and will normally not be more than (2 + <TT CLASS="VARNAME" >checkpoint_completion_target</TT >) * <TT CLASS="VARNAME" >checkpoint_segments</TT > + 1 or <TT CLASS="VARNAME" >checkpoint_segments</TT > + <A HREF="runtime-config-replication.html#GUC-WAL-KEEP-SEGMENTS" >wal_keep_segments</A > + 1 files. Each segment file is normally 16 MB (though this size can be altered when building the server). You can use this to estimate space requirements for <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM >. Ordinarily, when old log segment files are no longer needed, they are recycled (renamed to become the next segments in the numbered sequence). If, due to a short-term peak of log output rate, there are more than 3 * <TT CLASS="VARNAME" >checkpoint_segments</TT > + 1 segment files, the unneeded segment files will be deleted instead of recycled until the system gets back under this limit. </P ><P > In archive recovery or standby mode, the server periodically performs <I CLASS="FIRSTTERM" >restartpoints</I > which are similar to checkpoints in normal operation: the server forces all its state to disk, updates the <TT CLASS="FILENAME" >pg_control</TT > file to indicate that the already-processed WAL data need not be scanned again, and then recycles any old log segment files in <TT CLASS="FILENAME" >pg_xlog</TT > directory. A restartpoint is triggered if at least one checkpoint record has been replayed and <TT CLASS="VARNAME" >checkpoint_timeout</TT > seconds have passed since last restartpoint. In standby mode, a restartpoint is also triggered if <TT CLASS="VARNAME" >checkpoint_segments</TT > log segments have been replayed since last restartpoint and at least one checkpoint record has been replayed. Restartpoints can't be performed more frequently than checkpoints in the master because restartpoints can only be performed at checkpoint records. </P ><P > There are two commonly used internal <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > functions: <CODE CLASS="FUNCTION" >LogInsert</CODE > and <CODE CLASS="FUNCTION" >LogFlush</CODE >. <CODE CLASS="FUNCTION" >LogInsert</CODE > is used to place a new record into the <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > buffers in shared memory. If there is no space for the new record, <CODE CLASS="FUNCTION" >LogInsert</CODE > will have to write (move to kernel cache) a few filled <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > buffers. This is undesirable because <CODE CLASS="FUNCTION" >LogInsert</CODE > is used on every database low level modification (for example, row insertion) at a time when an exclusive lock is held on affected data pages, so the operation needs to be as fast as possible. What is worse, writing <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > buffers might also force the creation of a new log segment, which takes even more time. Normally, <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > buffers should be written and flushed by a <CODE CLASS="FUNCTION" >LogFlush</CODE > request, which is made, for the most part, at transaction commit time to ensure that transaction records are flushed to permanent storage. On systems with high log output, <CODE CLASS="FUNCTION" >LogFlush</CODE > requests might not occur often enough to prevent <CODE CLASS="FUNCTION" >LogInsert</CODE > from having to do writes. On such systems one should increase the number of <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > buffers by modifying the configuration parameter <A HREF="runtime-config-wal.html#GUC-WAL-BUFFERS" >wal_buffers</A >. When <A HREF="runtime-config-wal.html#GUC-FULL-PAGE-WRITES" >full_page_writes</A > is set and the system is very busy, setting this value higher will help smooth response times during the period immediately following each checkpoint. </P ><P > The <A HREF="runtime-config-wal.html#GUC-COMMIT-DELAY" >commit_delay</A > parameter defines for how many microseconds the server process will sleep after writing a commit record to the log with <CODE CLASS="FUNCTION" >LogInsert</CODE > but before performing a <CODE CLASS="FUNCTION" >LogFlush</CODE >. This delay allows other server processes to add their commit records to the log so as to have all of them flushed with a single log sync. No sleep will occur if <A HREF="runtime-config-wal.html#GUC-FSYNC" >fsync</A > is not enabled, or if fewer than <A HREF="runtime-config-wal.html#GUC-COMMIT-SIBLINGS" >commit_siblings</A > other sessions are currently in active transactions; this avoids sleeping when it's unlikely that any other session will commit soon. Note that on most platforms, the resolution of a sleep request is ten milliseconds, so that any nonzero <TT CLASS="VARNAME" >commit_delay</TT > setting between 1 and 10000 microseconds would have the same effect. Good values for these parameters are not yet clear; experimentation is encouraged. </P ><P > The <A HREF="runtime-config-wal.html#GUC-WAL-SYNC-METHOD" >wal_sync_method</A > parameter determines how <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > will ask the kernel to force <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > updates out to disk. All the options should be the same in terms of reliability, with the exception of <TT CLASS="LITERAL" >fsync_writethrough</TT >, which can sometimes force a flush of the disk cache even when other options do not do so. However, it's quite platform-specific which one will be the fastest; you can test option speeds using the <A HREF="pgtestfsync.html" ><SPAN CLASS="APPLICATION" >pg_test_fsync</SPAN ></A > module. Note that this parameter is irrelevant if <TT CLASS="VARNAME" >fsync</TT > has been turned off. </P ><P > Enabling the <A HREF="runtime-config-developer.html#GUC-WAL-DEBUG" >wal_debug</A > configuration parameter (provided that <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > has been compiled with support for it) will result in each <CODE CLASS="FUNCTION" >LogInsert</CODE > and <CODE CLASS="FUNCTION" >LogFlush</CODE > <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > call being logged to the server log. This option might be replaced by a more general mechanism in the future. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="wal-async-commit.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="wal-internals.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Asynchronous Commit</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="wal.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >WAL Internals</TD ></TR ></TABLE ></DIV ></BODY ></HTML >