PG_RESETXLOG(1) PostgreSQL 9.3.2 Documentation PG_RESETXLOG(1)NAMEpg_resetxlog - reset the write-ahead log and other control information
of a PostgreSQL database cluster
SYNOPSISpg_resetxlog [-f] [-n] [-o oid] [-x xid] [-e xid_epoch] [-m mxid,mxid]
[-O mxoff] [-l xlogfile] datadir
DESCRIPTIONpg_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, and -l options allow the next OID, next
transaction ID, next transaction ID's epoch, next and oldest
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:
· 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.
· 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 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 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.
PostgreSQL 9.3.2 2013 PG_RESETXLOG(1)
