1PG_RESETXLOG(1)         PostgreSQL Server Applications         PG_RESETXLOG(1)
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NAME

6       pg_resetxlog  - reset the write-ahead log and other control information
7       of a PostgreSQL database cluster
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SYNOPSIS

10       pg_resetxlog [ -f ]  [ -n ]  [ -ooid  ]  [ -x xid  ]  [ -e xid_epoch  ]
11       [ -m mxid  ]  [ -O mxoff  ]  [ -l timelineid,fileid,seg  ]  datadir
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DESCRIPTION

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.
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20       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.
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26       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.
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31       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.
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45       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:
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52       · 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).
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60       · 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.
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67       · 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.
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74       · 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.
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84         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.
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92       · 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.
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96       · 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.
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103       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.
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NOTES

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.
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118Application                       2014-02-17                   PG_RESETXLOG(1)
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