1PG_RESETXLOG(1) PostgreSQL Server Applications PG_RESETXLOG(1)
2
6 pg_resetxlog - reset the write-ahead log and other control information
7 of a PostgreSQL database cluster
8
10 pg_resetxlog [ -f ] [ -n ] [ -ooid ] [ -x xid ] [ -e xid_epoch ]
11 [ -m mxid ] [ -O mxoff ] [ -l timelineid,fileid,seg ] datadir
12
14 pg_resetxlog clears the write-ahead log (WAL) and optionally resets
15 some other control information stored in the pg_control file. This
16 function is sometimes needed if these files have become corrupted. It
17 should be used only as a last resort, when the server will not start
18 due to such corruption.
19 After running this command, it should be possible to start the server,
21 but bear in mind that the database might contain inconsistent data due
22 to partially-committed transactions. You should immediately dump your
23 data, run initdb, and reload. After reload, check for inconsistencies
24 and repair as needed.
25 This utility can only be run by the user who installed the server,
27 because it requires read/write access to the data directory. For
28 safety reasons, you must specify the data directory on the command
29 line. pg_resetxlog does not use the environment variable PGDATA.
30 If pg_resetxlog complains that it cannot determine valid data for
32 pg_control, you can force it to proceed anyway by specifying the -f
33 (force) switch. In this case plausible values will be substituted for
34 the missing data. Most of the fields can be expected to match, but man‐
35 ual assistance might be needed for the next OID, next transaction ID
36 and epoch, next multitransaction ID and offset, and WAL starting
37 address fields. These fields can be set using the switches discussed
38 below. If you are not able to determine correct values for all these
39 fields, -f can still be used, but the recovered database must be
40 treated with even more suspicion than usual: an immediate dump and
41 reload is imperative. Do not execute any data-modifying operations in
42 the database before you dump, as any such action is likely to make the
43 corruption worse.
44 The -o, -x, -e, -m, -O, and -l switches allow the next OID, next trans‐
46 action ID, next transaction ID's epoch, next multitransaction ID, next
47 multitransaction offset, and WAL starting address values to be set man‐
48 ually. These are only needed when pg_resetxlog is unable to determine
49 appropriate values by reading pg_control. Safe values can be determined
50 as follows:
51 · A safe value for the next transaction ID (-x) can be determined by
53 looking for the numerically largest file name in the directory
54 pg_clog under the data directory, adding one, and then multiplying by
55 1048576. Note that the file names are in hexadecimal. It is usually
56 easiest to specify the switch value in hexadecimal too. For example,
57 if 0011 is the largest entry in pg_clog, -x 0x1200000 will work (five
58 trailing zeroes provide the proper multiplier).
59 · A safe value for the next multitransaction ID (-m) can be determined
61 by looking for the numerically largest file name in the directory
62 pg_multixact/offsets under the data directory, adding one, and then
63 multiplying by 65536. As above, the file names are in hexadecimal, so
64 the easiest way to do this is to specify the switch value in hexadec‐
65 imal and add four zeroes.
66 · A safe value for the next multitransaction offset (-O) can be deter‐
68 mined by looking for the numerically largest file name in the direc‐
69 tory pg_multixact/members under the data directory, adding one, and
70 then multiplying by 65536. As above, the file names are in hexadeci‐
71 mal, so the easiest way to do this is to specify the switch value in
72 hexadecimal and add four zeroes.
73 · The WAL starting address (-l) should be larger than any WAL segment
75 file name currently existing in the directory pg_xlog under the data
76 directory. These names are also in hexadecimal and have three parts.
77 The first part is the ``timeline ID'' and should usually be kept the
78 same. Do not choose a value larger than 255 (0xFF) for the third
79 part; instead increment the second part and reset the third part to
80 0. For example, if 00000001000000320000004A is the largest entry in
81 pg_xlog, -l 0x1,0x32,0x4B will work; but if the largest entry is
82 000000010000003A000000FF, choose -l 0x1,0x3B,0x0 or more.
83 Note: pg_resetxlog itself looks at the files in pg_xlog and chooses a
85 default -l setting beyond the last existing file name. Therefore,
86 manual adjustment of -l should only be needed if you are aware of WAL
87 segment files that are not currently present in pg_xlog, such as
88 entries in an offline archive; or if the contents of pg_xlog have
89 been lost entirely.
90 · There is no comparably easy way to determine a next OID that's beyond
93 the largest one in the database, but fortunately it is not critical
94 to get the next-OID setting right.
95 · The transaction ID epoch is not actually stored anywhere in the data‐
97 base except in the field that is set by pg_resetxlog, so any value
98 will work so far as the database itself is concerned. You might need
99 to adjust this value to ensure that replication systems such as
100 Slony-I work correctly — if so, an appropriate value should be
101 obtainable from the state of the downstream replicated database.
102 The -n (no operation) switch instructs pg_resetxlog to print the values
104 reconstructed from pg_control and then exit without modifying anything.
105 This is mainly a debugging tool, but can be useful as a sanity check
106 before allowing pg_resetxlog to proceed for real.
107
109 This command must not be used when the server is running. pg_resetxlog
110 will refuse to start up if it finds a server lock file in the data
111 directory. If the server crashed then a lock file might have been left
112 behind; in that case you can remove the lock file to allow pg_resetxlog
113 to run. But before you do so, make doubly certain that there is no
114 server process still alive.
115Application 2011-09-22 PG_RESETXLOG(1)