Description
pg_resetxlog clears the write-ahead log and
optionally resets some fields 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,
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 datadir.
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) 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, WAL starting address, and database locale fields.
The first three 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.
The -o, -x, and -l switches allow
the next OID, next transaction ID, 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. A safe value for the
next transaction ID may be determined by looking for the largest
file name in $PGDATA/pg_clog, 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 0x1200000 will work (five trailing
zeroes provide the proper multiplier).
The WAL starting address should be
larger than any file number currently existing in
$PGDATA/pg_xlog. These also are in hex, and
have two parts. For example, if 000000FF0000003A is the
largest entry in pg_xlog, -l 0xFF,0x3B will work.
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 -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
before allowing pg_resetxlog to proceed for real.
