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The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 488 pages
ISBN 9781906966058
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3.4 pg_resetxlog

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] [-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 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 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 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:

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 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.

ISBN 9781906966058The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command ReferenceSee the print edition