pg\_resetwal

pg\_resetwal

1

Application

pg\_resetwal

reset the write-ahead log and other control information of a

PostgreSQL

database cluster

pg\_resetwal

-f

\--force

-n

\--dry-run

option

-D

\--pgdata

datadir

Description {#r1-app-pgresetwal-1}
===========

`pg_resetwal` 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_resetwal` does not use the environment variable PGDATA.

If `pg_resetwal` 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
location 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.

Options
=======

`-f`; `--force`

:   Force `pg_resetwal` to proceed even if it cannot determine valid
    data for `pg_control`, as explained above.

`-n`; `--dry-run`

:   The `-n`/`--dry-run` option instructs `pg_resetwal` 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_resetwal` to proceed for real.

`-V`; `--version`

:   Display version information, then exit.

`-?`; `--help`

:   Show help, then exit.

The following options are only needed when `pg_resetwal` is unable to
determine appropriate values by reading `pg_control`. Safe values can be
determined as described below. For values that take numeric arguments,
hexadecimal values can be specified by using the prefix `0x`.

`-c xid,xid`; `--commit-timestamp-ids=xid,xid`

:   Manually set the oldest and newest transaction IDs for which the
    commit time can be retrieved.

    A safe value for the oldest transaction ID for which the commit time
    can be retrieved (first part) 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) can be determined by looking for the numerically greatest file
    name in the same directory. The file names are in hexadecimal.

`-e xid_epoch`; `--epoch=xid_epoch`

:   Manually set the next transaction ID\'s epoch.

    The transaction ID epoch is not actually stored anywhere in the
    database except in the field that is set by `pg_resetwal`, 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 MDASH if so, an
    appropriate value should be obtainable from the state of the
    downstream replicated database.

`-l walfile`; `--next-wal-file=walfile`

:   Manually set the WAL starting location by specifying the name of the
    next WAL segment file.

    The name of next WAL segment file should be larger than any WAL
    segment file name currently existing in the directory `pg_wal` 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_wal`, use `-l 00000001000000320000004B` or
    higher.

    Note that when using nondefault WAL segment sizes, the numbers in
    the WAL file names are different from the LSNs that are reported by
    system functions and system views. This option takes a WAL file
    name, not an LSN.

    > **Note**
    >
    > `pg_resetwal` itself looks at the files in `pg_wal` 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_wal`, such as entries in an offline archive; or if the
    > contents of `pg_wal` have been lost entirely.

`-m mxid,mxid`; `--multixact-ids=mxid,mxid`

:   Manually set the next and oldest multitransaction ID.

    A safe value for the next multitransaction ID (first part) 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 (0x10000). 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. 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.

`-o oid`; `--next-oid=oid`

:   Manually set the next OID.

    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 mxoff`; `--multixact-offset=mxoff`

:   Manually set the next multitransaction offset.

    A safe value 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 (0xCC80).
    The file names are in hexadecimal. There is no simple recipe such as
    the ones for other options of appending zeroes.

`--wal-segsize=wal_segment_size`

:   Set the new WAL segment size, in megabytes. The value must be set to
    a power of 2 between 1 and 1024 (megabytes). See the same option of
    [???](#app-initdb) for more information.

    > **Note**
    >
    > While `pg_resetwal` will set the WAL starting address beyond the
    > latest existing WAL segment file, some segment size changes can
    > cause previous WAL file names to be reused. It is recommended to
    > use `-l` together with this option to manually set the WAL
    > starting address if WAL file name overlap will cause problems with
    > your archiving strategy.

`-u xid`; `--oldest-transaction-id=xid`

:   Manually set the oldest unfrozen transaction ID.

    A safe value can be determined by looking for the numerically
    smallest file name in the directory `pg_xact` under the data
    directory and then multiplying by 1048576 (0x100000). Note that the
    file names are in hexadecimal. It is usually easiest to specify the
    option value in hexadecimal too. For example, if `0007` is the
    smallest entry in `pg_xact`, `-u 0x700000` will work (five trailing
    zeroes provide the proper multiplier).

`-x xid`; `--next-transaction-id=xid`

:   Manually set the next transaction ID.

    A safe value can be determined by looking for the numerically
    largest file name in the directory `pg_xact` under the data
    directory, adding one, and then multiplying by 1048576 (0x100000).
    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_xact`, `-x 0x1200000` will work
    (five trailing zeroes provide the proper multiplier).

Environment
===========

PG\_COLOR

:   Specifies whether to use color in diagnostic messages. Possible
    values are `always`, `auto` and `never`.

Notes
=====

This command must not be used when the server is running. `pg_resetwal`
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_resetwal`
to run. But before you do so, make doubly certain that there is no
server process still alive.

`pg_resetwal` works only with servers of the same major version.

See Also
========
