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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 ]  [ -ooid  ]	[ -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 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  and
       epoch,  next  multitransaction ID and offset, WAL starting address, and
       database locale fields.	The first six 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, -e, -m, -O, and -l switches allow the next OID, next trans‐
       action  ID, next transaction ID's epoch, next multitransaction ID, next
       multitransaction offset, and WAL starting address values to be set man‐
       ually.  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 transaction ID epoch is not actually stored anywhere in the data‐
	 base 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) 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
       server process still alive.

Application			  2008-01-03		       PG_RESETXLOG(1)
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