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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 time-
lineid,fileid,seg ] datadir
DESCRIPTION
pg_resetxlog clears the write-ahead log (WAL) and optionally resets some other control information stored in the pg_con-
trol 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 spec-
ifying 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 corruption worse.
The -o, -x, -e, -m, -O, and -l switches allow the next OID, next transaction ID, next transaction ID's epoch, next multi-
transaction 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 fol-
lows:
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 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 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 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 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 cur-
rently 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 with-
out 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 2011-09-22 PG_RESETXLOG(1)
