<!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 8.0.11 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Write-Ahead Logging (WAL)" HREF="wal.html"><LINK REL="PREVIOUS" TITLE="Write-Ahead Logging (WAL)" HREF="wal.html"><LINK REL="NEXT" TITLE="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="2007-02-02T03:57:22"></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" >PostgreSQL 8.0.11 Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="wal.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="wal.html" >Fast Backward</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 25. Write-Ahead Logging (<ACRONYM CLASS="ACRONYM" >WAL</ACRONYM >)</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A HREF="wal.html" >Fast Forward</A ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="top" ><A 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" >25.2. <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" >Section 16.4</A > for general information about setting server configuration parameters. </P ><P > <I CLASS="FIRSTTERM" >Checkpoints</I ><A NAME="AEN22264" ></A > are points in the sequence of transactions at which it is guaranteed that the data files have been updated with all information logged 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. As a result, in the event of a crash, the crash recovery procedure knows from what point in the log (known as the redo record) it should start the REDO operation, since any changes made to data files before that point are already on disk. After a checkpoint has been made, any log segments written before 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 server's background writer process will automatically perform a checkpoint every so often. A checkpoint is created every <A HREF="runtime-config.html#GUC-CHECKPOINT-SEGMENTS" >checkpoint_segments</A > log segments, or every <A HREF="runtime-config.html#GUC-CHECKPOINT-TIMEOUT" >checkpoint_timeout</A > seconds, whichever comes first. The default settings are 3 segments and 300 seconds respectively. 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 be done 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. In addition, to ensure data page consistency, the first modification of a data page after each checkpoint results in logging the entire page content. Thus 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.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. </P ><P > There will be at least one WAL segment file, and will normally not be more than 2 * <TT CLASS="VARNAME" >checkpoint_segments</TT > + 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 2 * <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 > There are two commonly used <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 may 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 may 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.html#GUC-WAL-BUFFERS" >wal_buffers</A >. The default number of <ACRONYM CLASS="ACRONYM" >WAL</ACRONYM > buffers is 8. Increasing this value will correspondingly increase shared memory usage. (It should be noted that there is presently little evidence to suggest that increasing <TT CLASS="VARNAME" >wal_buffers</TT > beyond the default is worthwhile.) </P ><P > The <A HREF="runtime-config.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.html#GUC-FSYNC" >fsync</A > is not enabled, nor if fewer than <A HREF="runtime-config.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.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 as far as reliability goes, but it's quite platform-specific which one will be the fastest. Note that this parameter is irrelevant if <TT CLASS="VARNAME" >fsync</TT > has been turned off. </P ><P > Enabling the <A HREF="runtime-config.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 may 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.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" >Write-Ahead Logging (<ACRONYM CLASS="ACRONYM" >WAL</ACRONYM >)</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="wal.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Internals</TD ></TR ></TABLE ></DIV ></BODY ></HTML >