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CentOS 7.0 - man page for pg_resetxlog (centos section 1)

PG_RESETXLOG(1) 		  PostgreSQL 9.2.7 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] [-O mxoff]
		    [-l timelineid,fileid,seg] 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 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 (-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. As above, the file names are in
	   hexadecimal, so the easiest way to do this is to specify the option value in
	   hexadecimal and add 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 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 add four 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. Do not choose a value larger than 255 (0xFF) for the
	   third part; instead increment the second part and reset the third part to 0. For
	   example, if 00000001000000320000004A is the largest entry in pg_xlog, -l 0x1,0x32,0x4B
	   will work; but if the largest entry is 000000010000003A000000FF, choose -l
	   0x1,0x3B,0x0 or more.

	       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 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 9.2.7			    2014-02-17				  PG_RESETXLOG(1)


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