doc/src/sgml/ref/pg_resetwal.sgml

   1 <!--
   2 doc/src/sgml/ref/pg_resetwal.sgml
   3 PostgreSQL documentation
   4 -->
   5
   6 <refentry id="app-pgresetwal">
   7  <indexterm zone="app-pgresetwal">
   8   <primary>pg_resetwal</primary>
   9  </indexterm>
  10
  11  <refmeta>
  12   <refentrytitle><application>pg_resetwal</application></refentrytitle>
  13   <manvolnum>1</manvolnum>
  14   <refmiscinfo>Application</refmiscinfo>
  15  </refmeta>
  16
  17  <refnamediv>
  18   <refname>pg_resetwal</refname>
  19   <refpurpose>reset the write-ahead log and other control information of a <productname>PostgreSQL</productname> database cluster</refpurpose>
  20  </refnamediv>
  21
  22  <refsynopsisdiv>
  23   <cmdsynopsis>
  24    <command>pg_resetwal</command>
  25    <group choice="opt">
  26     <arg choice="plain"><option>-f</option></arg>
  27     <arg choice="plain"><option>--force</option></arg>
  28    </group>
  29    <group choice="opt">
  30     <arg choice="plain"><option>-n</option></arg>
  31     <arg choice="plain"><option>--dry-run</option></arg>
  32    </group>
  33    <arg rep="repeat"><replaceable>option</replaceable></arg>
  34    <group choice="plain">
  35     <group choice="opt">
  36      <arg choice="plain"><option>-D</option></arg>
  37      <arg choice="plain"><option>--pgdata</option></arg>
  38     </group>
  39     <replaceable class="parameter">datadir</replaceable>
  40    </group>
  41   </cmdsynopsis>
  42  </refsynopsisdiv>
  43
  44  <refsect1 id="r1-app-pgresetwal-1">
  45   <title>Description</title>
  46   <para>
  47    <command>pg_resetwal</command> clears the write-ahead log (WAL) and
  48    optionally resets some other control information stored in the
  49    <filename>pg_control</filename> file.  This function is sometimes needed
  50    if these files have become corrupted.  It should be used only as a
  51    last resort, when the server will not start due to such corruption.
  52   </para>
  53
  54   <para>
  55    Some options, such as <option>--wal-segsize</option> (see below), can also
  56    be used to modify certain global settings of a database cluster without the
  57    need to rerun <command>initdb</command>.  This can be done safely on an
  58    otherwise sound database cluster, if none of the dangerous modes mentioned
  59    below are used.
  60   </para>
  61
  62   <para>
  63    If <command>pg_resetwal</command> is used on a data directory where the
  64    server has been cleanly shut down and the control file is sound, then it
  65    will have no effect on the contents of the database system, except that no
  66    longer used WAL files are cleared away.  Any other use is potentially
  67    dangerous and must be done with great care.  <command>pg_resetwal</command>
  68    will require the <option>-f</option> (force) option to be specified before
  69    working on a data directory in an unclean shutdown state or with a
  70    corrupted control file.
  71   </para>
  72
  73   <para>
  74    After running this command on a data directory with corrupted WAL or a
  75    corrupted control file, it should be possible to start the server,
  76    but bear in mind that the database might contain inconsistent data due to
  77    partially-committed transactions.  You should immediately dump your data,
  78    run <command>initdb</command>, and restore.  After restore, check for
  79    inconsistencies and repair as needed.
  80   </para>
  81
  82   <para>
  83    If <command>pg_resetwal</command> complains that it cannot determine
  84    valid data for <filename>pg_control</filename>, you can force it to proceed anyway
  85    by specifying the <option>-f</option> (force) option.  In this case plausible
  86    values will be substituted for the missing data.  Most of the fields can be
  87    expected to match, but manual assistance might be needed for the next OID,
  88    next transaction ID and epoch, next multitransaction ID and offset, and
  89    WAL starting location fields. These fields can be set using the options
  90    discussed below. If you are not able to determine correct values for all
  91    these fields, <option>-f</option> can still be used, but
  92    the recovered database must be treated with even more suspicion than
  93    usual: an immediate dump and restore is imperative.  <emphasis>Do not</emphasis>
  94    execute any data-modifying operations in the database before you dump,
  95    as any such action is likely to make the corruption worse.
  96   </para>
  97
  98   <para>
  99    This utility can only be run by the user who installed the server, because
 100    it requires read/write access to the data directory.
 101   </para>
 102  </refsect1>
 103
 104  <refsect1>
 105   <title>Options</title>
 106
 107   <variablelist>
 108    <varlistentry>
 109     <term><replaceable class="parameter">datadir</replaceable></term>
 110     <term><option>-D <replaceable class="parameter">datadir</replaceable></option></term>
 111     <term><option>--pgdata=<replaceable class="parameter">datadir</replaceable></option></term>
 112     <listitem>
 113      <para>
 114       Specifies the location of the database directory.
 115       For safety reasons, you must specify the data directory on the command
 116       line.  <command>pg_resetwal</command> does not use the environment
 117       variable <envar>PGDATA</envar>.
 118      </para>
 119     </listitem>
 120    </varlistentry>
 121
 122    <varlistentry>
 123     <term><option>-f</option></term>
 124     <term><option>--force</option></term>
 125     <listitem>
 126      <para>
 127       Force <command>pg_resetwal</command> to proceed even in situations where
 128       it could be dangerous, as explained above.  Specifically, this option is
 129       required to proceed if the server had not been cleanly shut down or if
 130       <command>pg_resetwal</command> cannot determine valid data for
 131       <filename>pg_control</filename>.
 132      </para>
 133     </listitem>
 134    </varlistentry>
 135
 136    <varlistentry>
 137     <term><option>-n</option></term>
 138     <term><option>--dry-run</option></term>
 139     <listitem>
 140      <para>
 141       The <option>-n</option>/<option>--dry-run</option> option instructs
 142       <command>pg_resetwal</command> to print the values reconstructed from
 143       <filename>pg_control</filename> and values about to be changed, and then exit
 144       without modifying anything. This is mainly a debugging tool, but can be
 145       useful as a sanity check before allowing <command>pg_resetwal</command>
 146       to proceed for real.
 147      </para>
 148     </listitem>
 149    </varlistentry>
 150
 151    <varlistentry>
 152     <term><option>-V</option></term>
 153     <term><option>--version</option></term>
 154     <listitem><para>Display version information, then exit.</para></listitem>
 155    </varlistentry>
 156
 157    <varlistentry>
 158     <term><option>-?</option></term>
 159     <term><option>--help</option></term>
 160     <listitem><para>Show help, then exit.</para></listitem>
 161    </varlistentry>
 162   </variablelist>
 163
 164   <para>
 165    The following options are only needed when
 166    <command>pg_resetwal</command> is unable to determine appropriate values
 167    by reading <filename>pg_control</filename>.  Safe values can be determined as
 168    described below.  For values that take numeric arguments, hexadecimal
 169    values can be specified by using the prefix <literal>0x</literal>.
 170   </para>
 171
 172   <variablelist>
 173    <varlistentry>
 174     <term><option>-c <replaceable class="parameter">xid</replaceable>,<replaceable class="parameter">xid</replaceable></option></term>
 175     <term><option>--commit-timestamp-ids=<replaceable class="parameter">xid</replaceable>,<replaceable class="parameter">xid</replaceable></option></term>
 176     <listitem>
 177      <para>
 178       Manually set the oldest and newest transaction IDs for which the commit
 179       time can be retrieved.
 180      </para>
 181
 182      <para>
 183       A safe value for the oldest transaction ID for which the commit time can
 184       be retrieved (first part) can be determined by looking
 185       for the numerically smallest file name in the directory
 186       <filename>pg_commit_ts</filename> under the data directory.  Conversely, a safe
 187       value for the newest transaction ID for which the commit time can be
 188       retrieved (second part) can be determined by looking for the numerically
 189       greatest file name in the same directory.  The file names are in
 190       hexadecimal.
 191      </para>
 192     </listitem>
 193    </varlistentry>
 194
 195    <varlistentry>
 196     <term><option>-e <replaceable class="parameter">xid_epoch</replaceable></option></term>
 197     <term><option>--epoch=<replaceable class="parameter">xid_epoch</replaceable></option></term>
 198     <listitem>
 199      <para>
 200       Manually set the next transaction ID's epoch.
 201      </para>
 202
 203      <para>
 204       The transaction ID epoch is not actually stored anywhere in the database
 205       except in the field that is set by <command>pg_resetwal</command>,
 206       so any value will work so far as the database itself is concerned.
 207       You might need to adjust this value to ensure that replication
 208       systems such as <application>Slony-I</application> and
 209       <application>Skytools</application> work correctly &mdash;
 210       if so, an appropriate value should be obtainable from the state of
 211       the downstream replicated database.
 212      </para>
 213     </listitem>
 214    </varlistentry>
 215
 216    <varlistentry>
 217     <term><option>-l <replaceable class="parameter">walfile</replaceable></option></term>
 218     <term><option>--next-wal-file=<replaceable class="parameter">walfile</replaceable></option></term>
 219     <listitem>
 220      <para>
 221       Manually set the WAL starting location by specifying the name of the
 222       next WAL segment file.
 223      </para>
 224
 225      <para>
 226       The name of next WAL segment file should be
 227       larger than any WAL segment file name currently existing in
 228       the directory <filename>pg_wal</filename> under the data directory.
 229       These names are also in hexadecimal and have three parts.  The first
 230       part is the <quote>timeline ID</quote> and should usually be kept the same.
 231       For example, if <filename>00000001000000320000004A</filename> is the
 232       largest entry in <filename>pg_wal</filename>, use <literal>-l 00000001000000320000004B</literal> or higher.
 233      </para>
 234
 235      <para>
 236       Note that when using nondefault WAL segment sizes, the numbers in the WAL
 237       file names are different from the LSNs that are reported by system
 238       functions and system views.  This option takes a WAL file name, not an
 239       LSN.
 240      </para>
 241
 242      <note>
 243       <para>
 244        <command>pg_resetwal</command> itself looks at the files in
 245        <filename>pg_wal</filename> and chooses a default <option>-l</option> setting
 246        beyond the last existing file name.  Therefore, manual adjustment of
 247        <option>-l</option> should only be needed if you are aware of WAL segment
 248        files that are not currently present in <filename>pg_wal</filename>, such as
 249        entries in an offline archive; or if the contents of
 250        <filename>pg_wal</filename> have been lost entirely.
 251       </para>
 252      </note>
 253     </listitem>
 254    </varlistentry>
 255
 256    <varlistentry>
 257     <term><option>-m <replaceable class="parameter">mxid</replaceable>,<replaceable class="parameter">mxid</replaceable></option></term>
 258     <term><option>--multixact-ids=<replaceable class="parameter">mxid</replaceable>,<replaceable class="parameter">mxid</replaceable></option></term>
 259     <listitem>
 260      <para>
 261       Manually set the next and oldest multitransaction ID.
 262      </para>
 263
 264      <para>
 265       A safe value for the next multitransaction ID (first part) can be
 266       determined by looking for the numerically largest file name in the
 267       directory <filename>pg_multixact/offsets</filename> under the data directory,
 268       adding one, and then multiplying by 65536 (0x10000).  Conversely, a safe
 269       value for the oldest multitransaction ID (second part of
 270       <option>-m</option>) can be determined by looking for the numerically smallest
 271       file name in the same directory and multiplying by 65536.  The file
 272       names are in hexadecimal, so the easiest way to do this is to specify
 273       the option value in hexadecimal and append four zeroes.
 274      </para>
 275     </listitem>
 276    </varlistentry>
 277
 278    <varlistentry>
 279     <term><option>-o <replaceable class="parameter">oid</replaceable></option></term>
 280     <term><option>--next-oid=<replaceable class="parameter">oid</replaceable></option></term>
 281     <listitem>
 282      <para>
 283       Manually set the next OID.
 284      </para>
 285
 286      <para>
 287       There is no comparably easy way to determine a next OID that's beyond
 288       the largest one in the database, but fortunately it is not critical to
 289       get the next-OID setting right.
 290      </para>
 291     </listitem>
 292    </varlistentry>
 293
 294    <varlistentry>
 295     <term><option>-O <replaceable class="parameter">mxoff</replaceable></option></term>
 296     <term><option>--multixact-offset=<replaceable class="parameter">mxoff</replaceable></option></term>
 297     <listitem>
 298      <para>
 299       Manually set the next multitransaction offset.
 300      </para>
 301
 302      <para>
 303       A safe value can be determined by looking for the numerically largest
 304       file name in the directory <filename>pg_multixact/members</filename> under the
 305       data directory, adding one, and then multiplying by 52352 (0xCC80).
 306       The file names are in hexadecimal.  There is no simple recipe such as
 307       the ones for other options of appending zeroes.
 308      </para>
 309     </listitem>
 310    </varlistentry>
 311
 312    <varlistentry>
 313     <term><option>--wal-segsize=<replaceable class="parameter">wal_segment_size</replaceable></option></term>
 314     <listitem>
 315      <para>
 316       Set the new WAL segment size, in megabytes.  The value must be set to a
 317       power of 2 between 1 and 1024 (megabytes).  See the same option of <xref
 318       linkend="app-initdb"/> for more information.
 319      </para>
 320
 321      <para>
 322       This option can also be used to change the WAL segment size of an
 323       existing database cluster, avoiding the need to
 324       re-<command>initdb</command>.
 325      </para>
 326
 327      <note>
 328       <para>
 329        While <command>pg_resetwal</command> will set the WAL starting address
 330        beyond the latest existing WAL segment file, some segment size changes
 331        can cause previous WAL file names to be reused.  It is recommended to
 332        use <option>-l</option> together with this option to manually set the
 333        WAL starting address if WAL file name overlap will cause problems with
 334        your archiving strategy.
 335       </para>
 336      </note>
 337     </listitem>
 338    </varlistentry>
 339
 340    <varlistentry>
 341     <term><option>-u <replaceable class="parameter">xid</replaceable></option></term>
 342     <term><option>--oldest-transaction-id=<replaceable class="parameter">xid</replaceable></option></term>
 343     <listitem>
 344      <para>
 345       Manually set the oldest unfrozen transaction ID.
 346      </para>
 347
 348      <para>
 349       A safe value can be determined by looking for the numerically smallest
 350       file name in the directory <filename>pg_xact</filename> under the data directory
 351       and then multiplying by 1048576 (0x100000).  Note that the file names are in
 352       hexadecimal.  It is usually easiest to specify the option value in
 353       hexadecimal too. For example, if <filename>0007</filename> is the smallest entry
 354       in <filename>pg_xact</filename>, <literal>-u 0x700000</literal> will work (five
 355       trailing zeroes provide the proper multiplier).
 356      </para>
 357     </listitem>
 358    </varlistentry>
 359
 360    <varlistentry>
 361     <term><option>-x <replaceable class="parameter">xid</replaceable></option></term>
 362     <term><option>--next-transaction-id=<replaceable class="parameter">xid</replaceable></option></term>
 363     <listitem>
 364      <para>
 365       Manually set the next transaction ID.
 366      </para>
 367
 368      <para>
 369       A safe value can be determined by looking for the numerically largest
 370       file name in the directory <filename>pg_xact</filename> under the data directory,
 371       adding one,
 372       and then multiplying by 1048576 (0x100000).  Note that the file names are in
 373       hexadecimal.  It is usually easiest to specify the option value in
 374       hexadecimal too. For example, if <filename>0011</filename> is the largest entry
 375       in <filename>pg_xact</filename>, <literal>-x 0x1200000</literal> will work (five
 376       trailing zeroes provide the proper multiplier).
 377      </para>
 378     </listitem>
 379    </varlistentry>
 380   </variablelist>
 381  </refsect1>
 382
 383  <refsect1>
 384   <title>Environment</title>
 385
 386   <variablelist>
 387    <varlistentry>
 388     <term><envar>PG_COLOR</envar></term>
 389     <listitem>
 390      <para>
 391       Specifies whether to use color in diagnostic messages. Possible values
 392       are <literal>always</literal>, <literal>auto</literal> and
 393       <literal>never</literal>.
 394      </para>
 395     </listitem>
 396    </varlistentry>
 397   </variablelist>
 398  </refsect1>
 399
 400  <refsect1>
 401   <title>Notes</title>
 402
 403   <para>
 404    This command must not be used when the server is
 405    running.  <command>pg_resetwal</command> will refuse to start up if
 406    it finds a server lock file in the data directory.  If the
 407    server crashed then a lock file might have been left
 408    behind; in that case you can remove the lock file to allow
 409    <command>pg_resetwal</command> to run.  But before you do
 410    so, make doubly certain that there is no server process still alive.
 411   </para>
 412
 413   <para>
 414    <command>pg_resetwal</command> works only with servers of the same
 415    major version.
 416   </para>
 417  </refsect1>
 418
 419  <refsect1>
 420   <title>See Also</title>
 421
 422   <simplelist type="inline">
 423    <member><xref linkend="app-pgcontroldata"/></member>
 424   </simplelist>
 425  </refsect1>
 426 </refentry>