pg_resetxlog man page on Raspbian

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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 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) 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 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  switches  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 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 can be determined
       as follows:

       · 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 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) 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 switch value in hexadec‐
	 imal and add four zeroes.

       · A safe value for the next multitransaction offset (-O) can 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 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.

       · 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 can 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 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.

Application			  2013-04-02		       PG_RESETXLOG(1)
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