pg_resetxlog
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 ] [ -l
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
manual assistance may be needed for the next OID, next transaction ID,
WAL starting address, and database locale fields. The first three 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, and -l switches allow the next OID, next transaction ID,
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. A safe value for the next transaction ID may be
determined by looking for the numerically largest file name in the
directory pg_clog under the data directory, adding one, and then mul-
tiplying 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). The WAL
starting address should be larger than any file number currently
existing in the directory pg_xlog under the data directory. The
addresses are also in hexadecimal and have two parts. For example, if
000000FF0000003A is the largest entry in pg_xlog, -l 0xFF,0x3B will
work. 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 val-
ues 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 2008-01-03 PG_RESETXLOG(1)
