|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
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pg_resetxlog -- reset the write-ahead log and other control information of a PostgreSQL database cluster
pg_resetxlog [-f] [-n] [-o oid] [-x xid] [-e xid_epoch] [-m mxid] [-O mxoff] [-l timelineid, fileid, seg] datadir
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 may contain inconsistent data due to
partially-committed transactions. You should immediately dump your data,
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
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 may be needed for the next OID,
next transaction ID and epoch, next multitransaction ID and offset,
WAL starting address, and database locale fields.
The first six of these can be set using the switches discussed below.
pg_resetxlog's own environment is the source for its
guess at the locale fields; take care that
LANG and so forth
match the environment that
initdb was run in.
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.
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 may be determined as
A safe value for the next transaction ID (
-x) may 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 switch value in hexadecimal too. For example, if ‘0011’ is the largest entry in ‘pg_clog’,
-x 0x1200000will work (five trailing zeroes provide the proper multiplier).
A safe value for the next multitransaction ID (
-m) may 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. As above, the file names are in hexadecimal, so the easiest way to do this is to specify the switch value in hexadecimal and add four zeroes.
A safe value for the next multitransaction offset (
-O) may 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 65536. As above, the file names are in hexadecimal, so the easiest way to do this is to specify the switch value in hexadecimal and add four zeroes.
The WAL starting address (
-l) should be larger than any 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. Do not choose a value larger than 255 (
0xFF) for the third part; instead increment the second part and reset the third part to 0. For example, if ‘00000001000000320000004A’ is the largest entry in ‘pg_xlog’,
-l 0x1,0x32,0x4Bwill work; but if the largest entry is ‘000000010000003A000000FF’, choose
-l 0x1,0x3B,0x0or more.
- 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.
-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 may be useful as a sanity check
pg_resetxlog to proceed for real.
This command must not be used when the server is
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 may 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 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|