pg_resetwal — reset the write-ahead log and other control information of a PostgreSQL database cluster
pg_resetwal [ --force | -f ] [ --dry-run | -n ] [option...] [ --pgdata | -D ] datadir
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.
-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--versionDisplay version information, then exit.
-?--helpShow 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,xidManually 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_epochManually 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 —
if so, an appropriate value should be obtainable from the state of
the downstream replicated database.
-l walfile--next-wal-file=walfileManually 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.
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,mxidManually 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=oidManually 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=mxoffManually 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_sizeSet 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 initdb for more information.
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.
-x xid--next-transaction-id=xidManually 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).
PG_COLOR
Specifies whether to use color in diagnostics messages. Possible values
are always, auto,
never.
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.