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PG_RESETXLOG(1)		PostgreSQL Server Applications	       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	 ]  [  -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 may 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) switch. In this case plausible values will be  substituted  for
       the missing data. Most of the fields can be expected to match, but man‐
       ual 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.  pg_resetxlog's own environment is the source for its
       guess at the locale fields; take care that LANG and so forth match  the
       environment  that  initdb was run in.  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, -m, -O, and -l switches allow the next OID,	next  transac‐
       tion  ID,  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 may be determined as follows:

       · A safe value for the next transaction ID (-x) may  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 switch 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).

       · A safe value for the next multitransaction ID (-m) may 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 switch value in hexadec‐
	 imal and add four zeroes.

       · A  safe value for the next multitransaction offset (-O) may be deter‐
	 mined by looking for the numerically largest file name in the	direc‐
	 tory  pg_multixact/members  under the data directory, adding one, and
	 then multiplying by 65536. As above, the file names are in  hexadeci‐
	 mal,  so the easiest way to do this is to specify the switch value in
	 hexadecimal and add four zeroes.

       · The WAL starting address (-l) should be larger	 than  any  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 exam‐
	 ple, 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.

       · 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.

       The -n (no operation) switch instructs pg_resetxlog to print the values
       reconstructed from pg_control and then exit without modifying anything.
       This is mainly a debugging tool, but may be useful as  a	 sanity	 check
       before allowing pg_resetxlog to proceed for real.

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 may 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
       postmaster nor any backend server process still alive.

Application			  2005-11-05		       PG_RESETXLOG(1)
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