pg_resetxlog - Online in the Cloud

PROGRAM:

NAME

pg_resetxlog - reset the write-ahead log and other control information of a PostgreSQL
database cluster

SYNOPSIS

pg_resetxlog [-c xid,xid] [-f] [-n] [-o oid] [-x xid] [-e xid_epoch] [-m mxid,mxid]
[-O mxoff] [-l xlogfile] {[-D] 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) option. 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 options 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, -c and -l options allow the next OID, next transaction ID, next
transaction ID's epoch, next and oldest multitransaction ID, next multitransaction offset,
oldest and newest transaction IDs for which the commit time can be retrieved, 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:

· 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 option 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 (first part of -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. Conversely, a
safe value for the oldest multitransaction ID (second part of -m) can be determined by
looking for the numerically smallest file name in the same directory and multiplying
by 65536. As above, the file names are in hexadecimal, so the easiest way to do this
is to specify the option value in hexadecimal and append four zeroes.

· 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 52352. As above, the file names
are in hexadecimal. There is no simple recipe such as the ones above of appending
zeroes.

· A safe value for the oldest transaction ID for which the commit time can be retrieved
(first part of -c) can be determined by looking for the numerically smallest file name
in the directory pg_commit_ts under the data directory. Conversely, a safe value for
the newest transaction ID for which the commit time can be retrieved (second part of
-c) can be determined by looking for the numerically greatest file name in the same
directory. As above, the file names are in hexadecimal.

· 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. For example, if 00000001000000320000004A is the
largest entry in pg_xlog, use -l 00000001000000320000004B or higher.

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 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 and Skytools work correctly — if so, an appropriate value
should be obtainable from the state of the downstream replicated database.

The -n (no operation) option instructs pg_resetxlog to print the values reconstructed from
pg_control and values about to be changed, 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.

The -V and --version options print the pg_resetxlog version and exit. The options -? and
--help show supported arguments, and exit.

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.

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