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<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/pg_resetxlog.sgml,v 1.12 2005/11/04 23:14:02 petere Exp $
PostgreSQL documentation
-->
<refentry id="APP-PGRESETXLOG">
<refmeta>
<refentrytitle id="APP-PGRESETXLOG-TITLE"><application>pg_resetxlog</application></refentrytitle>
<manvolnum>1</manvolnum>
<refmiscinfo>Application</refmiscinfo>
</refmeta>
<refnamediv>
<refname>pg_resetxlog</refname>
<refpurpose>reset the write-ahead log and other control information of a <productname>PostgreSQL</productname> database cluster</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>pg_resetxlog</command>
<arg> -f </arg>
<arg> -n </arg>
<arg> -o <replaceable class="parameter">oid</replaceable> </arg>
<arg> -x <replaceable class="parameter">xid</replaceable> </arg>
<arg> -m <replaceable class="parameter">mxid</replaceable> </arg>
<arg> -O <replaceable class="parameter">mxoff</replaceable> </arg>
<arg> -l <replaceable class="parameter">timelineid</replaceable>,<replaceable class="parameter">fileid</replaceable>,<replaceable class="parameter">seg</replaceable> </arg>
<arg choice="plain"><replaceable>datadir</replaceable></arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1 id="R1-APP-PGRESETXLOG-1">
<title>Description</title>
<para>
<command>pg_resetxlog</command> clears the write-ahead log (WAL) and
optionally resets some other control information stored in the
<filename>pg_control</> 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>
After running this command, it should be possible to start the server,
but bear in mind that the database may contain inconsistent data due to
partially-committed transactions. You should immediately dump your data,
run <command>initdb</>, and reload. After reload, check for
inconsistencies and repair as needed.
</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.
For safety reasons, you must specify the data directory on the command line.
<command>pg_resetxlog</command> does not use the environment variable
<envar>PGDATA</>.
</para>
<para>
If <command>pg_resetxlog</command> complains that it cannot determine
valid data for <filename>pg_control</>, you can force it to proceed anyway
by specifying the <literal>-f</> (force) switch. In this case plausible
values will be substituted for the missing data. Most of the fields can be
expected to match, but manual assistance may be needed for the next OID,
next transaction ID, next multitransaction ID and offset,
WAL starting address, and database locale fields.
The first five of these can be set using the switches discussed below.
<command>pg_resetxlog</command>'s own environment is the source for its
guess at the locale fields; take care that <envar>LANG</> and so forth
match the environment that <command>initdb</> was run in.
If you are not able to determine correct values for all these fields,
<literal>-f</> can still be used, but
the recovered database must be treated with even more suspicion than
usual: an immediate dump and reload is imperative. <emphasis>Do not</>
execute any data-modifying operations in the database before you dump;
as any such action is likely to make the corruption worse.
</para>
<para>
The <literal>-o</>, <literal>-x</>, <literal>-m</>, <literal>-O</>,
and <literal>-l</>
switches allow the next OID, next transaction ID, next multitransaction
ID, next multitransaction offset, and WAL starting address values to
be set manually. These are only needed when
<command>pg_resetxlog</command> is unable to determine appropriate values
by reading <filename>pg_control</>. Safe values may be determined as
follows:
<itemizedlist>
<listitem>
<para>
A safe value for the next transaction ID (<literal>-x</>)
may be determined by looking for the numerically largest
file name in the directory <filename>pg_clog</> under the data directory,
adding one,
and then multiplying by 1048576. Note that the file names are in
hexadecimal. It is usually easiest to specify the switch value in
hexadecimal too. For example, if <filename>0011</> is the largest entry
in <filename>pg_clog</>, <literal>-x 0x1200000</> will work (five
trailing zeroes provide the proper multiplier).
</para>
</listitem>
<listitem>
<para>
A safe value for the next multitransaction ID (<literal>-m</>)
may be determined by looking for the numerically largest
file name in the directory <filename>pg_multixact/offsets</> under the
data directory, adding one, and then multiplying by 65536. As above,
the file names are in hexadecimal, so the easiest way to do this is to
specify the switch value in hexadecimal and add four zeroes.
</para>
</listitem>
<listitem>
<para>
A safe value for the next multitransaction offset (<literal>-O</>)
may be determined by looking for the numerically largest
file name in the directory <filename>pg_multixact/members</> under the
data directory, adding one, and then multiplying by 65536. As above,
the file names are in hexadecimal, so the easiest way to do this is to
specify the switch value in hexadecimal and add four zeroes.
</para>
</listitem>
<listitem>
<para>
The WAL starting address (<literal>-l</>) should be
larger than any file name currently existing in
the directory <filename>pg_xlog</> under the data directory.
These names are also in hexadecimal and have three parts. The first
part is the <quote>timeline ID</> and should usually be kept the same.
Do not choose a value larger than 255 (<literal>0xFF</>) for the third
part; instead increment the second part and reset the third part to 0.
For example, if <filename>00000001000000320000004A</> is the
largest entry in <filename>pg_xlog</>, <literal>-l 0x1,0x32,0x4B</> will
work; but if the largest entry is
<filename>000000010000003A000000FF</>, choose <literal>-l 0x1,0x3B,0x0</>
or more.
</para>
</listitem>
<listitem>
<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>
</itemizedlist>
</para>
<para>
The <literal>-n</> (no operation) switch instructs
<command>pg_resetxlog</command> to print the values reconstructed from
<filename>pg_control</> and then exit without modifying anything.
This is mainly a debugging tool, but may be useful as a sanity check
before allowing <command>pg_resetxlog</command> to proceed for real.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
This command must not be used when the server is
running. <command>pg_resetxlog</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 may have been left
behind; in that case you can remove the lock file to allow
<command>pg_resetxlog</command> to run. But before you do
so, make doubly certain that there
is no <command>postmaster</command> nor any backend server process still alive.
</para>
</refsect1>
</refentry>
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