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 inconsis-
       tencies 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
       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 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  cor-
       ruption worse.

       The  -o,  -x,  -e,  -m, -O, and -l switches 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 switch value in hexadecimal too. For example, if 0011 is the largest entry in pg_clog, -x 0x1200000 will work (five trail-
	 ing 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 switch 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  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.

       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) switch instructs pg_resetxlog to print the values reconstructed from pg_control and then exit without modifying any-
       thing.  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							    2010-05-14							   PG_RESETXLOG(1)