pg_resetxlog(1)
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 manual 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 environ-
ment that initdb was run in. If you are not able to deter-
mine 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 mul-
titransaction ID, next multitransaction offset, and WAL
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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 may be determined
as follows:
o 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).
o 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 hexade-
cimal and add four zeroes.
o A safe value for the next multitransaction offset (-O) may
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 hexade-
cimal and add four zeroes.
o 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 hexade-
cimal and have three parts. The first part is the ``time-
line 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.
o There is no comparably easy way to determine a next OID
that's beyond the largest one in the database, but for-
tunately 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 data-
base itself is concerned. You might need to adjust this
value to ensure that replication systems such as Slony-I
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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 Last change: 2008-01-03 3
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