2001-03-16 06:44:33 +01:00
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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v 1.5 2001/03/16 05:44:33 tgl Exp $ -->
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2001-01-25 00:15:19 +01:00
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<chapter id="wal">
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<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>
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<note>
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<title>Author</title>
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<para>
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Vadim Mikheev and Oliver Elphick
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</para>
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</note>
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<sect1 id="wal-general">
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<title>General Description</Title>
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<para>
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<firstterm>Write Ahead Logging</firstterm> (<acronym>WAL</acronym>)
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is a standard approach to transaction logging. Its detailed
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description may be found in most (if not all) books about
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transaction processing. Briefly, <acronym>WAL</acronym>'s central
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concept is that changes to data files (where tables and indices
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reside) must be written only after those changes have been logged -
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that is, when log records have been flushed to permanent
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storage. When we follow this procedure, we do not need to flush
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data pages to disk on every transaction commit, because we know
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that in the event of a crash we will be able to recover the
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database using the log: any changes that have not been applied to
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the data pages will first be redone from the log records (this is
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roll-forward recovery, also known as REDO) and then changes made by
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uncommitted transactions will be removed from the data pages
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(roll-backward recovery - UNDO).
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</para>
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<sect2 id="wal-benefits-now">
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<title>Immediate Benefits of <acronym>WAL</acronym></title>
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<para>
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The first obvious benefit of using <acronym>WAL</acronym> is a
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significantly reduced number of disk writes, since only the log
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file needs to be flushed to disk at the time of transaction
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commit; in multi-user environments, commits of many transactions
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may be accomplished with a single <function>fsync()</function> of
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the log file. Furthermore, the log file is written sequentially,
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and so the cost of syncing the log is much less than the cost of
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flushing the data pages.
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</para>
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<para>
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The next benefit is consistency of the data pages. The truth is
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that, before <acronym>WAL</acronym>,
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<productname>PostgreSQL</productname> was never able to guarantee
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consistency in the case of a crash. Before
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<acronym>WAL</acronym>, any crash during writing could result in:
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<orderedlist>
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<listitem>
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<simpara>index tuples pointing to non-existent table rows</simpara>
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</listitem>
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<listitem>
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<simpara>index tuples lost in split operations</simpara>
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</listitem>
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<listitem>
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<simpara>totally corrupted table or index page content, because
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of partially written data pages</simpara>
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</listitem>
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</orderedlist>
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Problems with indices (problems 1 and 2) could possibly have been
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fixed by additional <function>fsync()</function> calls, but it is
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not obvious how to handle the last case without
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<acronym>WAL</acronym>; <acronym>WAL</acronym> saves the entire
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data page content in the log if that is required to ensure page
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consistency for after-crash recovery.
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</para>
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</sect2>
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<sect2 id="wal-benefits-later">
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<title>Future Benefits</title>
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<para>
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In this first release of <acronym>WAL</acronym>, UNDO operation is
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not implemented, because of lack of time. This means that changes
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made by aborted transactions will still occupy disk space and that
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we still need a permanent <filename>pg_log</filename> file to hold
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the status of transactions, since we are not able to re-use
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transaction identifiers. Once UNDO is implemented,
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<filename>pg_log</filename> will no longer be required to be
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permanent; it will be possible to remove
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<filename>pg_log</filename> at shutdown, split it into segments
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and remove old segments.
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</para>
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<para>
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With UNDO, it will also be possible to implement
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<firstterm>savepoints</firstterm> to allow partial rollback of
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invalid transaction operations (parser errors caused by mistyping
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commands, insertion of duplicate primary/unique keys and so on)
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with the ability to continue or commit valid operations made by
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the transaction before the error. At present, any error will
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invalidate the whole transaction and require a transaction abort.
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</para>
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<para>
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<acronym>WAL</acronym> offers the opportunity for a new method for
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database on-line backup and restore (<acronym>BAR</acronym>). To
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use this method, one would have to make periodic saves of data
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files to another disk, a tape or another host and also archive the
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<acronym>WAL</acronym> log files. The database file copy and the
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archived log files could be used to restore just as if one were
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restoring after a crash. Each time a new database file copy was
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made the old log files could be removed. Implementing this
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facility will require the logging of data file and index creation
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and deletion; it will also require development of a method for
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copying the data files (operating system copy commands are not
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suitable).
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</para>
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</sect2>
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</sect1>
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<sect1 id="wal-implementation">
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<title>Implementation</title>
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<para>
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<acronym>WAL</acronym> is automatically enabled from release 7.1
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onwards. No action is required from the administrator with the
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exception of ensuring that the additional disk-space requirements
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of the <acronym>WAL</acronym> logs are met, and that any necessary
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tuning is done (see <xref linkend="wal-configuration">).
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</para>
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<para>
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<acronym>WAL</acronym> logs are stored in the directory
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<Filename><replaceable>$PGDATA</replaceable>/pg_xlog</Filename>, as
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a set of segment files, each 16 MB in size. Each segment is
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divided into 8 kB pages. The log record headers are described in
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<filename>access/xlog.h</filename>; record content is dependent on
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the type of event that is being logged. Segment files are given
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sequential numbers as names, starting at
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<filename>0000000000000000</filename>. The numbers do not wrap, at
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present, but it should take a very long time to exhaust the
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available stock of numbers.
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</para>
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<para>
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The <acronym>WAL</acronym> buffers and control structure are in
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shared memory, and are handled by the backends; they are protected
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by spinlocks. The demand on shared memory is dependent on the
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number of buffers; the default size of the <acronym>WAL</acronym>
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buffers is 64 kB.
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</para>
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<para>
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It is of advantage if the log is located on another disk than the
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main database files. This may be achieved by moving the directory,
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<filename>pg_xlog</filename>, to another location (while the
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postmaster is shut down, of course) and creating a symbolic link
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from the original location in <replaceable>$PGDATA</replaceable> to
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the new location.
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</para>
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<para>
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The aim of <acronym>WAL</acronym>, to ensure that the log is
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written before database records are altered, may be subverted by
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disk drives that falsely report a successful write to the kernel,
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when, in fact, they have only cached the data and not yet stored it
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on the disk. A power failure in such a situation may still lead to
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irrecoverable data corruption; administrators should try to ensure
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that disks holding <productname>PostgreSQL</productname>'s data and
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log files do not make such false reports.
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</para>
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<sect2 id="wal-recovery">
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<title>Database Recovery with <acronym>WAL</acronym></title>
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<para>
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After a checkpoint has been made and the log flushed, the
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checkpoint's position is saved in the file
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<filename>pg_control</filename>. Therefore, when recovery is to be
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done, the backend first reads <filename>pg_control</filename> and
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then the checkpoint record; next it reads the redo record, whose
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position is saved in the checkpoint, and begins the REDO operation.
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Because the entire content of the pages is saved in the log on the
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first page modification after a checkpoint, the pages will be first
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restored to a consistent state.
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</para>
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<para>
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Using <filename>pg_control</filename> to get the checkpoint
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position speeds up the recovery process, but to handle possible
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corruption of <filename>pg_control</filename>, we should actually
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implement the reading of existing log segments in reverse order --
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newest to oldest -- in order to find the last checkpoint. This has
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not yet been done in release 7.1.
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</para>
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</sect2>
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</sect1>
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<sect1 id="wal-configuration">
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<title><acronym>WAL</acronym> Configuration</title>
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<para>
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There are several <acronym>WAL</acronym>-related parameters that
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affect database performance. This section explains their use.
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Consult <xref linkend="runtime-config"> for details about setting
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configuration parameters.
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</para>
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<para>
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There are two commonly used <acronym>WAL</acronym> functions:
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<function>LogInsert</function> and <function>LogFlush</function>.
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<function>LogInsert</function> is used to place a new record into
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the <acronym>WAL</acronym> buffers in shared memory. If there is no
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space for the new record, <function>LogInsert</function> will have
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to write (move to kernel cache) a few filled <acronym>WAL</acronym>
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buffers. This is undesirable because <function>LogInsert</function>
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is used on every database low level modification (for example,
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tuple insertion) at a time when an exclusive lock is held on
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affected data pages and the operation is supposed to be as fast as
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possible; what is worse, writing <acronym>WAL</acronym> buffers may
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also cause the creation of a new log segment, which takes even more
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time. Normally, <acronym>WAL</acronym> buffers should be written
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and flushed by a <function>LogFlush</function> request, which is
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made, for the most part, at transaction commit time to ensure that
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transaction records are flushed to permanent storage. On systems
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with high log output, <function>LogFlush</function> requests may
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not occur often enough to prevent <acronym>WAL</acronym> buffers
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being written by <function>LogInsert</function>. On such systems
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one should increase the number of <acronym>WAL</acronym> buffers by
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modifying the <varname>WAL_BUFFERS</varname> parameter. The default
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number of <acronym>WAL</acronym> buffers is 8. Increasing this
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value will have an impact on shared memory usage.
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</para>
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<para>
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<firstterm>Checkpoints</firstterm> are points in the sequence of
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transactions at which it is guaranteed that the data files have
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been updated with all information logged before the checkpoint. At
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checkpoint time, all dirty data pages are flushed to disk and a
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special checkpoint record is written to the log file. As result, in
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the event of a crash, the recoverer knows from what record in the
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log (known as the redo record) it should start the REDO operation,
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since any changes made to data files before that record are already
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on disk. After a checkpoint has been made, any log segments written
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before the redo record are removed, so checkpoints are used to free
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disk space in the <acronym>WAL</acronym> directory. (When
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<acronym>WAL</acronym>-based <acronym>BAR</acronym> is implemented,
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the log segments can be archived instead of just being removed.)
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The checkpoint maker is also able to create a few log segments for
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future use, so as to avoid the need for
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<function>LogInsert</function> or <function>LogFlush</function> to
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spend time in creating them.
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</para>
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<para>
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The <acronym>WAL</acronym> log is held on the disk as a set of 16
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MB files called <firstterm>segments</firstterm>. By default a new
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segment is created only if more than 75% of the current segment is
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XLOG (and related) changes:
* Store two past checkpoint locations, not just one, in pg_control.
On startup, we fall back to the older checkpoint if the newer one
is unreadable. Also, a physical copy of the newest checkpoint record
is kept in pg_control for possible use in disaster recovery (ie,
complete loss of pg_xlog). Also add a version number for pg_control
itself. Remove archdir from pg_control; it ought to be a GUC
parameter, not a special case (not that it's implemented yet anyway).
* Suppress successive checkpoint records when nothing has been entered
in the WAL log since the last one. This is not so much to avoid I/O
as to make it actually useful to keep track of the last two
checkpoints. If the things are right next to each other then there's
not a lot of redundancy gained...
* Change CRC scheme to a true 64-bit CRC, not a pair of 32-bit CRCs
on alternate bytes. Polynomial borrowed from ECMA DLT1 standard.
* Fix XLOG record length handling so that it will work at BLCKSZ = 32k.
* Change XID allocation to work more like OID allocation. (This is of
dubious necessity, but I think it's a good idea anyway.)
* Fix a number of minor bugs, such as off-by-one logic for XLOG file
wraparound at the 4 gig mark.
* Add documentation and clean up some coding infelicities; move file
format declarations out to include files where planned contrib
utilities can get at them.
* Checkpoint will now occur every CHECKPOINT_SEGMENTS log segments or
every CHECKPOINT_TIMEOUT seconds, whichever comes first. It is also
possible to force a checkpoint by sending SIGUSR1 to the postmaster
(undocumented feature...)
* Defend against kill -9 postmaster by storing shmem block's key and ID
in postmaster.pid lockfile, and checking at startup to ensure that no
processes are still connected to old shmem block (if it still exists).
* Switch backends to accept SIGQUIT rather than SIGUSR1 for emergency
stop, for symmetry with postmaster and xlog utilities. Clean up signal
handling in bootstrap.c so that xlog utilities launched by postmaster
will react to signals better.
* Standalone bootstrap now grabs lockfile in target directory, as added
insurance against running it in parallel with live postmaster.
2001-03-13 02:17:06 +01:00
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used. One can instruct the server to pre-create up to 64 log segments
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2001-01-25 00:15:19 +01:00
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at checkpoint time by modifying the <varname>WAL_FILES</varname>
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configuration parameter.
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</para>
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<para>
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For faster after-crash recovery, it would be better to create
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checkpoints more often. However, one should balance this against
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the cost of flushing dirty data pages; in addition, to ensure data
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page consistency, the first modification of a data page after each
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checkpoint results in logging the entire page content, thus
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increasing output to log and the log's size.
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</para>
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<para>
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XLOG (and related) changes:
* Store two past checkpoint locations, not just one, in pg_control.
On startup, we fall back to the older checkpoint if the newer one
is unreadable. Also, a physical copy of the newest checkpoint record
is kept in pg_control for possible use in disaster recovery (ie,
complete loss of pg_xlog). Also add a version number for pg_control
itself. Remove archdir from pg_control; it ought to be a GUC
parameter, not a special case (not that it's implemented yet anyway).
* Suppress successive checkpoint records when nothing has been entered
in the WAL log since the last one. This is not so much to avoid I/O
as to make it actually useful to keep track of the last two
checkpoints. If the things are right next to each other then there's
not a lot of redundancy gained...
* Change CRC scheme to a true 64-bit CRC, not a pair of 32-bit CRCs
on alternate bytes. Polynomial borrowed from ECMA DLT1 standard.
* Fix XLOG record length handling so that it will work at BLCKSZ = 32k.
* Change XID allocation to work more like OID allocation. (This is of
dubious necessity, but I think it's a good idea anyway.)
* Fix a number of minor bugs, such as off-by-one logic for XLOG file
wraparound at the 4 gig mark.
* Add documentation and clean up some coding infelicities; move file
format declarations out to include files where planned contrib
utilities can get at them.
* Checkpoint will now occur every CHECKPOINT_SEGMENTS log segments or
every CHECKPOINT_TIMEOUT seconds, whichever comes first. It is also
possible to force a checkpoint by sending SIGUSR1 to the postmaster
(undocumented feature...)
* Defend against kill -9 postmaster by storing shmem block's key and ID
in postmaster.pid lockfile, and checking at startup to ensure that no
processes are still connected to old shmem block (if it still exists).
* Switch backends to accept SIGQUIT rather than SIGUSR1 for emergency
stop, for symmetry with postmaster and xlog utilities. Clean up signal
handling in bootstrap.c so that xlog utilities launched by postmaster
will react to signals better.
* Standalone bootstrap now grabs lockfile in target directory, as added
insurance against running it in parallel with live postmaster.
2001-03-13 02:17:06 +01:00
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The postmaster spawns a special backend process every so often
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to create the next checkpoint. A checkpoint is created every
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<varname>CHECKPOINT_SEGMENTS</varname> log segments, or every
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<varname>CHECKPOINT_TIMEOUT</varname> seconds, whichever comes first.
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The default settings are 3 segments and 300 seconds respectively.
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It is also possible to force a checkpoint by using the SQL command
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2001-01-25 00:15:19 +01:00
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<command>CHECKPOINT</command>.
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</para>
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<para>
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2001-02-18 05:50:43 +01:00
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The <varname>COMMIT_DELAY</varname> parameter defines for how many
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microseconds the backend will sleep after writing a commit
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record to the log with <function>LogInsert</function> but before
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2001-01-25 00:15:19 +01:00
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performing a <function>LogFlush</function>. This delay allows other
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backends to add their commit records to the log so as to have all
|
2001-02-26 01:50:08 +01:00
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of them flushed with a single log sync. No sleep will occur if fsync
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is not enabled or if fewer than <varname>COMMIT_SIBLINGS</varname>
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other backends are not currently in active transactions; this avoids
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sleeping when it's unlikely that any other backend will commit soon.
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Note that on most platforms, the resolution of a sleep request is
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ten milliseconds, so that any nonzero <varname>COMMIT_DELAY</varname>
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setting between 1 and 10000 microseconds will have the same effect.
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2001-01-25 00:15:19 +01:00
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|
</para>
|
2001-03-16 06:44:33 +01:00
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<para>
|
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|
The <varname>WAL_SYNC_METHOD</varname> parameter determines how
|
|
|
|
Postgres will ask the kernel to force WAL updates out to disk.
|
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|
All the options should be the same as far as reliability goes,
|
|
|
|
but it's quite platform-specific which one will be the fastest.
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|
Note that this parameter is irrelevant if <varname>FSYNC</varname>
|
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|
has been turned off.
|
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|
|
</para>
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|
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|
<para>
|
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Setting the <varname>WAL_DEBUG</varname> parameter to any non-zero
|
|
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|
value will result in each <function>LogInsert</function> and
|
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|
|
<function>LogFlush</function> <acronym>WAL</acronym> call being
|
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|
|
logged to standard error. At present, it makes no difference what
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the non-zero value is. This option may be replaced by a more
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general mechanism in the future.
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</para>
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2001-01-25 00:15:19 +01:00
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</sect1>
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</chapter>
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Local variables:
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"./reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:("/usr/lib/sgml/catalog")
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sgml-local-ecat-files:nil
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End:
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-->
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