File: gdbm.info,  Node: Manual crash recovery,  Next: Performance Impact,  Prev: Crash recovery,  Up: Crash Tolerance

17.5 Manual crash recovery
==========================

"Manual recovery" is usually performed with the help of the 'gdbmtool'
utility.  Start 'gdbmtool' in read-only mode (the '-r') option.  Once in
the command shell, issue the following command:

     snapshot A B

where A and B are names of the two snapshot files you configured using
the 'gdbm_failure_atomic' function.  This command investigates both
files and prints out detailed diagnostics.

   Its output begins with a line listing one of the error codes above,
followed by a colon and a textual description of the error.  The lines
that follow show details for each snapshot file.

   Each snapshot description begins with the snapshot file name followed
by a colon and four fields, in this order:

  1. File permission bits in octal.
  2. File permission bits in 'ls -l' notation.
  3. Modification timestamp.
  4. Numsync counter.  For databases in standard 'GDBM' format, this
     field is 'N/A'.  If the counter cannot be obtained because of
     error, this field is '?'.

   Any errors or inconsistencies discovered are reported in the lines
that follow, one error per line.  Here's an example of the 'snapshot'
command output, describing the 'GDBM_SNAPSHOT_ERR' condition:

     gdbmtool> snapshot even.dbf odd.dbf
     GDBM_SNAPSHOT_ERR: Error selecting snapshot.
     even.dbf: 200 -w------- 1627820627.485681330 ?
     odd.dbf: 600 rw------- 1627820627.689503918 301
     odd.dbf: ERROR: bad file mode

   Line 2 lists the meta-data of the snapshot 'even.dbf'.  The 'numsync'
field contains question mark because the file permissions (write-only)
prevented 'gdbmtool' from opening it.

   The lines for 'odd.dbf' show the actual reason for the error: bad
file mode (read-write).  Apparently, the file mode has been changed
manually after the crash.  The timestamp of the file, which is more
recent than that of 'even.dbf', suggests that it might be used for
recovery.  To confirm this guess, change the mode of the 'even.dbf' to
read-only and repeat the 'snapshot' command:

     gdbmtool> ! chmod 400 even.dbf
     gdbmtool> snapshot even.dbf odd.dbf
     GDBM_SNAPSHOT_ERR: Error selecting snapshot.
     even.dbf: 400 r-------- 1627820627.485681330 300
     odd.dbf: 600 rw------- 1627820627.689503918 301
     odd.dbf: ERROR: bad file mode

   This shows the numsync value of the 'even.dbf' file, which is exactly
one less than that of 'odd.dbf'.  This means that the latter should be
selected for recovery.

   For completeness sake, you can change the mode of 'odd.dbf' to
read-only as well and repeat the 'snapshot' command.  In this case you
will see:

     gdbmtool> ! chmod 400 odd.dbf
     gdbmtool> snapshot even.dbf odd.dbf
     GDBM_SNAPSHOT_OK: Selected the most recent snapshot.
     odd.dbf: 400 r-------- 1627820627.689503918 301