PG_RESETWAL(1) PostgreSQL 10.1 Documentation PG_RESETWAL(1)NAMEpg_resetwal - reset the write-ahead log and other control information
of a PostgreSQL database cluster
SYNOPSISpg_resetwal [-f] [-n] [option...] {[-D] datadir}
DESCRIPTIONpg_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
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.
OPTIONS-f
Force pg_resetwal to proceed even if it cannot determine valid data
for pg_control, as explained above.
-n
The -n (no operation) 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
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
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 — if so, an appropriate
value should be obtainable from the state of the downstream
replicated database.
-l walfile
Manually set the WAL starting address.
The WAL starting address 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
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
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
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
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.
-x 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).
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 ALSOpg_controldata(1)PostgreSQL 10.1 2017 PG_RESETWAL(1)
