PG_RESETXLOG(1)         PostgreSQL 9.4.5 Documentation         PG_RESETXLOG(1)



NAME
       pg_resetxlog - reset the write-ahead log and other control information
       of a PostgreSQL database cluster

SYNOPSIS
       pg_resetxlog [-f] [-n] [-o oid] [-x xid] [-e xid_epoch] [-m mxid,mxid]
                    [-O mxoff] [-l xlogfile] datadir

DESCRIPTION
       pg_resetxlog clears the write-ahead log (WAL) and optionally resets
       some other control information stored in the 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.

       After running this command, 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 initdb, and reload. After reload, check for inconsistencies
       and repair as needed.

       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.
       pg_resetxlog does not use the environment variable PGDATA.

       If pg_resetxlog complains that it cannot determine valid data for
       pg_control, you can force it to proceed anyway by specifying the -f
       (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
       address fields. These fields can be set using the options discussed
       below. If you are not able to determine correct values for all these
       fields, -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.  Do not execute any data-modifying operations in
       the database before you dump, as any such action is likely to make the
       corruption worse.

       The -o, -x, -e, -m, -O, and -l options allow the next OID, next
       transaction ID, next transaction ID's epoch, next and oldest
       multitransaction ID, next multitransaction offset, and WAL starting
       address values to be set manually. These are only needed when
       pg_resetxlog is unable to determine appropriate values by reading
       pg_control. Safe values can be determined as follows:

       o   A safe value for the next transaction ID (-x) can be determined by
           looking for the numerically largest file name in the directory
           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 option value in hexadecimal too. For
           example, if 0011 is the largest entry in pg_clog, -x 0x1200000 will
           work (five trailing zeroes provide the proper multiplier).

       o   A safe value for the next multitransaction ID (first part of -m)
           can be determined by looking for the numerically largest file name
           in the directory pg_multixact/offsets under the data directory,
           adding one, and then multiplying by 65536. Conversely, a safe value
           for the oldest multitransaction ID (second part of -m) can be
           determined by looking for the numerically smallest file name in the
           same directory and multiplying by 65536. As above, 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.

       o   A safe value for the next multitransaction offset (-O) can be
           determined by looking for the numerically largest file name in the
           directory pg_multixact/members under the data directory, adding
           one, and then multiplying by 52352. As above, the file names are in
           hexadecimal. There is no simple recipe such as the ones above of
           appending zeroes.

       o   The WAL starting address (-l) should be larger than any WAL segment
           file name currently existing in the directory pg_xlog under the
           data directory. These names are also in hexadecimal and have three
           parts. The first part is the "timeline ID" and should usually be
           kept the same. For example, if 00000001000000320000004A is the
           largest entry in pg_xlog, use -l 00000001000000320000004B or
           higher.

               Note
               pg_resetxlog itself looks at the files in pg_xlog and chooses a
               default -l setting beyond the last existing file name.
               Therefore, manual adjustment of -l should only be needed if you
               are aware of WAL segment files that are not currently present
               in pg_xlog, such as entries in an offline archive; or if the
               contents of pg_xlog have been lost entirely.

       o   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.

       o   The transaction ID epoch is not actually stored anywhere in the
           database except in the field that is set by pg_resetxlog, 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 Slony-I work correctly -- if so, an appropriate value
           should be obtainable from the state of the downstream replicated
           database.

       The -n (no operation) option instructs pg_resetxlog to print the values
       reconstructed from pg_control 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 pg_resetxlog to proceed
       for real.

       The -V and --version options print the pg_resetxlog version and exit.
       The options -?  and --help show supported arguments, and exit.

NOTES
       This command must not be used when the server is running.  pg_resetxlog
       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 pg_resetxlog
       to run. But before you do so, make doubly certain that there is no
       server process still alive.



PostgreSQL 9.4.5                     2015                      PG_RESETXLOG(1)
