<!--
doc/src/sgml/ref/pg_resetwal.sgml
PostgreSQL documentation
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<refentry id="app-pgresetwal">
 <indexterm zone="app-pgresetwal">
  <primary>pg_resetwal</primary>
 </indexterm>

 <refmeta>
  <refentrytitle><application>pg_resetwal</application></refentrytitle>
  <manvolnum>1</manvolnum>
  <refmiscinfo>Application</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>pg_resetwal</refname>
  <refpurpose>reset the write-ahead log and other control information of a <productname>PostgreSQL</productname> database cluster</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
  <cmdsynopsis>
   <command>pg_resetwal</command>
   <group choice="opt">
    <arg choice="plain"><option>-f</option></arg>
    <arg choice="plain"><option>--force</option></arg>
   </group>
   <group choice="opt">
    <arg choice="plain"><option>-n</option></arg>
    <arg choice="plain"><option>--dry-run</option></arg>
   </group>
   <arg rep="repeat"><replaceable>option</replaceable></arg>
   <group choice="plain">
    <group choice="opt">
     <arg choice="plain"><option>-D</option></arg>
     <arg choice="plain"><option>--pgdata</option></arg>
    </group>
    <replaceable class="parameter">datadir</replaceable>
   </group>
  </cmdsynopsis>
 </refsynopsisdiv>

 <refsect1 id="r1-app-pgresetwal-1">
  <title>Description</title>
  <para>
   <command>pg_resetwal</command> clears the write-ahead log (WAL) and
   optionally resets some other control information stored in the
   <filename>pg_control</filename> file.  This function is sometimes needed
   if these files have become corrupted.  It should be used only as a
   last resort, when the server will not start due to such corruption.
  </para>

  <para>
   Some options, such as <option>--wal-segsize</option> (see below), can also
   be used to modify certain global settings of a database cluster without the
   need to rerun <command>initdb</command>.  This can be done safely on an
   otherwise sound database cluster, if none of the dangerous modes mentioned
   below are used.
  </para>

  <para>
   If <command>pg_resetwal</command> is used on a data directory where the
   server has been cleanly shut down and the control file is sound, then it
   will have no effect on the contents of the database system, except that no
   longer used WAL files are cleared away.  Any other use is potentially
   dangerous and must be done with great care.  <command>pg_resetwal</command>
   will require the <option>-f</option> (force) option to be specified before
   working on a data directory in an unclean shutdown state or with a
   corrupted control file.
  </para>

  <para>
   After running this command on a data directory with corrupted WAL or a
   corrupted control file, it should be possible to start the server,
   but bear in mind that the database might contain inconsistent data due to
   partially-committed transactions.  You should immediately dump your data,
   run <command>initdb</command>, and restore.  After restore, check for
   inconsistencies and repair as needed.
  </para>

  <para>
   If <command>pg_resetwal</command> complains that it cannot determine
   valid data for <filename>pg_control</filename>, you can force it to proceed anyway
   by specifying the <option>-f</option> (force) option.  In this case plausible
   values will be substituted for the missing data.  Most of the fields can be
   expected to match, but manual assistance might be needed for the next OID,
   next transaction ID and epoch, next multitransaction ID and offset, and
   WAL starting location fields. These fields can be set using the options
   discussed below. If you are not able to determine correct values for all
   these fields, <option>-f</option> can still be used, but
   the recovered database must be treated with even more suspicion than
   usual: an immediate dump and restore is imperative.  <emphasis>Do not</emphasis>
   execute any data-modifying operations in the database before you dump,
   as any such action is likely to make the corruption worse.
  </para>

  <para>
   This utility can only be run by the user who installed the server, because
   it requires read/write access to the data directory.
  </para>
 </refsect1>

 <refsect1>
  <title>Options</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">datadir</replaceable></term>
    <term><option>-D <replaceable class="parameter">datadir</replaceable></option></term>
    <term><option>--pgdata=<replaceable class="parameter">datadir</replaceable></option></term>
    <listitem>
     <para>
      Specifies the location of the database directory.
      For safety reasons, you must specify the data directory on the command
      line.  <command>pg_resetwal</command> does not use the environment
      variable <envar>PGDATA</envar>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-f</option></term>
    <term><option>--force</option></term>
    <listitem>
     <para>
      Force <command>pg_resetwal</command> to proceed even in situations where
      it could be dangerous, as explained above.  Specifically, this option is
      required to proceed if the server had not been cleanly shut down or if
      <command>pg_resetwal</command> cannot determine valid data for
      <filename>pg_control</filename>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-n</option></term>
    <term><option>--dry-run</option></term>
    <listitem>
     <para>
      The <option>-n</option>/<option>--dry-run</option> option instructs
      <command>pg_resetwal</command> to print the values reconstructed from
      <filename>pg_control</filename> and values about to be changed, and then exit
      without modifying anything. This is mainly a debugging tool, but can be
      useful as a sanity check before allowing <command>pg_resetwal</command>
      to proceed for real.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-V</option></term>
    <term><option>--version</option></term>
    <listitem><para>Display version information, then exit.</para></listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-?</option></term>
    <term><option>--help</option></term>
    <listitem><para>Show help, then exit.</para></listitem>
   </varlistentry>
  </variablelist>

  <para>
   The following options are only needed when
   <command>pg_resetwal</command> is unable to determine appropriate values
   by reading <filename>pg_control</filename>.  Safe values can be determined as
   described below.  For values that take numeric arguments, hexadecimal
   values can be specified by using the prefix <literal>0x</literal>.  Note
   that these instructions only apply with the standard block size of 8 kB.
  </para>

  <variablelist>
   <varlistentry>
    <term><option>-c <replaceable class="parameter">xid</replaceable>,<replaceable class="parameter">xid</replaceable></option></term>
    <term><option>--commit-timestamp-ids=<replaceable class="parameter">xid</replaceable>,<replaceable class="parameter">xid</replaceable></option></term>
    <listitem>
     <para>
      Manually set the oldest and newest transaction IDs for which the commit
      time can be retrieved.
     </para>

     <para>
      A safe value for the oldest transaction ID for which the commit time can
      be retrieved (first part) can be determined by looking
      for the numerically smallest file name in the directory
      <filename>pg_commit_ts</filename> under the data directory.  Conversely, a safe
      value for the newest transaction ID for which the commit time can be
      retrieved (second part) can be determined by looking for the numerically
      greatest file name in the same directory.  The file names are in
      hexadecimal.
     </para>
      <!-- XXX: Should there be a multiplier, similar to the other options? -->
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-e <replaceable class="parameter">xid_epoch</replaceable></option></term>
    <term><option>--epoch=<replaceable class="parameter">xid_epoch</replaceable></option></term>
    <listitem>
     <para>
      Manually set the next transaction ID's epoch.
     </para>

     <para>
      The transaction ID epoch is not actually stored anywhere in the database
      except in the field that is set by <command>pg_resetwal</command>,
      so any value will work so far as the database itself is concerned.
      You might need to adjust this value to ensure that replication
      systems such as <application>Slony-I</application> and
      <application>Skytools</application> work correctly &mdash;
      if so, an appropriate value should be obtainable from the state of
      the downstream replicated database.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-l <replaceable class="parameter">walfile</replaceable></option></term>
    <term><option>--next-wal-file=<replaceable class="parameter">walfile</replaceable></option></term>
    <listitem>
     <para>
      Manually set the WAL starting location by specifying the name of the
      next WAL segment file.
     </para>

     <para>
      The name of next WAL segment file should be
      larger than any WAL segment file name currently existing in
      the directory <filename>pg_wal</filename> under the data directory.
      These names are also in hexadecimal and have three parts.  The first
      part is the <quote>timeline ID</quote> and should usually be kept the same.
      For example, if <filename>00000001000000320000004A</filename> is the
      largest entry in <filename>pg_wal</filename>, use <literal>-l 00000001000000320000004B</literal> or higher.
     </para>

     <para>
      Note that when using nondefault WAL segment sizes, the numbers in the WAL
      file names are different from the LSNs that are reported by system
      functions and system views.  This option takes a WAL file name, not an
      LSN.
     </para>

     <note>
      <para>
       <command>pg_resetwal</command> itself looks at the files in
       <filename>pg_wal</filename> and chooses a default <option>-l</option> setting
       beyond the last existing file name.  Therefore, manual adjustment of
       <option>-l</option> should only be needed if you are aware of WAL segment
       files that are not currently present in <filename>pg_wal</filename>, such as
       entries in an offline archive; or if the contents of
       <filename>pg_wal</filename> have been lost entirely.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-m <replaceable class="parameter">mxid</replaceable>,<replaceable class="parameter">mxid</replaceable></option></term>
    <term><option>--multixact-ids=<replaceable class="parameter">mxid</replaceable>,<replaceable class="parameter">mxid</replaceable></option></term>
    <listitem>
     <para>
      Manually set the next and oldest multitransaction ID.
     </para>

     <para>
      A safe value for the next multitransaction ID (first part) can be
      determined by looking for the numerically largest file name in the
      directory <filename>pg_multixact/offsets</filename> under the data directory,
      adding one, and then multiplying by 65536 (0x10000).  Conversely, a safe
      value for the oldest multitransaction ID (second part of
      <option>-m</option>) can be determined by looking for the numerically smallest
      file name in the same directory and multiplying by 65536.  The file
      names are in hexadecimal, so the easiest way to do this is to specify
      the option value in hexadecimal and append four zeroes.
     </para>
     <!-- 65536 = SLRU_PAGES_PER_SEGMENT * BLCKSZ / sizeof(MultiXactOffset) -->
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-o <replaceable class="parameter">oid</replaceable></option></term>
    <term><option>--next-oid=<replaceable class="parameter">oid</replaceable></option></term>
    <listitem>
     <para>
      Manually set the next OID.
     </para>

     <para>
      There is no comparably easy way to determine a next OID that's beyond
      the largest one in the database, but fortunately it is not critical to
      get the next-OID setting right.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-O <replaceable class="parameter">mxoff</replaceable></option></term>
    <term><option>--multixact-offset=<replaceable class="parameter">mxoff</replaceable></option></term>
    <listitem>
     <para>
      Manually set the next multitransaction offset.
     </para>

     <para>
      A safe value can be determined by looking for the numerically largest
      file name in the directory <filename>pg_multixact/members</filename> under the
      data directory, adding one, and then multiplying by 52352 (0xCC80).
      The file names are in hexadecimal.  There is no simple recipe such as
      the ones for other options of appending zeroes.
     </para>
     <!-- 52352 = SLRU_PAGES_PER_SEGMENT * floor(BLCKSZ/20) * 4; see multixact.c -->
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-u <replaceable class="parameter">xid</replaceable></option></term>
    <term><option>--oldest-transaction-id=<replaceable class="parameter">xid</replaceable></option></term>
    <listitem>
     <para>
      Manually set the oldest unfrozen transaction ID.
     </para>

     <para>
      A safe value can be determined by looking for the numerically smallest
      file name in the directory <filename>pg_xact</filename> under the data directory
      and then multiplying by 1048576 (0x100000).  Note that the file names are in
      hexadecimal.  It is usually easiest to specify the option value in
      hexadecimal too. For example, if <filename>0007</filename> is the smallest entry
      in <filename>pg_xact</filename>, <literal>-u 0x700000</literal> will work (five
      trailing zeroes provide the proper multiplier).
     </para>
     <!-- 1048576 = SLRU_PAGES_PER_SEGMENT * BLCKSZ * CLOG_XACTS_PER_BYTE -->
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>-x <replaceable class="parameter">xid</replaceable></option></term>
    <term><option>--next-transaction-id=<replaceable class="parameter">xid</replaceable></option></term>
    <listitem>
     <para>
      Manually set the next transaction ID.
     </para>

     <para>
      A safe value can be determined by looking for the numerically largest
      file name in the directory <filename>pg_xact</filename> under the data directory,
      adding one,
      and then multiplying by 1048576 (0x100000).  Note that the file names are in
      hexadecimal.  It is usually easiest to specify the option value in
      hexadecimal too. For example, if <filename>0011</filename> is the largest entry
      in <filename>pg_xact</filename>, <literal>-x 0x1200000</literal> will work (five
      trailing zeroes provide the proper multiplier).
     </para>
     <!-- 1048576 = SLRU_PAGES_PER_SEGMENT * BLCKSZ * CLOG_XACTS_PER_BYTE -->
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>--char-signedness=<replaceable class="parameter">option</replaceable></option></term>
    <listitem>
     <para>
      Manually set the default char signedness. Possible values are
      <literal>signed</literal> and <literal>unsigned</literal>.
     </para>
     <para>
      For a database cluster that <command>pg_upgrade</command> upgraded from
      a <productname>PostgreSQL</productname> version before 18, the safe
      value would be the default <type>char</type> signedness of the platform
      that ran the cluster before that upgrade. For all other
      clusters, <literal>signed</literal> would be the safe value. However,
      this option is exclusively for use with <command>pg_upgrade</command>
      and should not normally be used manually.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><option>--wal-segsize=<replaceable class="parameter">wal_segment_size</replaceable></option></term>
    <listitem>
     <para>
      Set the new WAL segment size, in megabytes.  The value must be set to a
      power of 2 between 1 and 1024 (megabytes).  See the same option of <xref
      linkend="app-initdb"/> for more information.
     </para>

     <para>
      This option can also be used to change the WAL segment size of an
      existing database cluster, avoiding the need to
      re-<command>initdb</command>.
     </para>

     <note>
      <para>
       While <command>pg_resetwal</command> will set the WAL starting address
       beyond the latest existing WAL segment file, some segment size changes
       can cause previous WAL file names to be reused.  It is recommended to
       use <option>-l</option> together with this option to manually set the
       WAL starting address if WAL file name overlap will cause problems with
       your archiving strategy.
      </para>
     </note>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Environment</title>

  <variablelist>
   <varlistentry>
    <term><envar>PG_COLOR</envar></term>
    <listitem>
     <para>
      Specifies whether to use color in diagnostic messages. Possible values
      are <literal>always</literal>, <literal>auto</literal> and
      <literal>never</literal>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   This command must not be used when the server is
   running.  <command>pg_resetwal</command> will refuse to start up if
   it finds a server lock file in the data directory.  If the
   server crashed then a lock file might have been left
   behind; in that case you can remove the lock file to allow
   <command>pg_resetwal</command> to run.  But before you do
   so, make doubly certain that there is no server process still alive.
  </para>

  <para>
   <command>pg_resetwal</command> works only with servers of the same
   major version.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="app-pgcontroldata"/></member>
  </simplelist>
 </refsect1>
</refentry>
