document how do deal with WAL-MISSING-AFTER warnings

See also the "Direct restore procedure" troubleshooting section, which

also applies here.

Dealing with Nagios warnings

----------------------------

Monitoring warnings

-------------------

TODO: there's some more information about backup handling in the

[Debian DSA documentation](https://dsa.debian.org/howto/postgres-backup/).

### WAL-MISSING-AFTER

Example message:

    [troodi, main] WAL-MISSING-AFTER: troodi/main.WAL.00000001000000D9000000AD

This means that a WAL file is missing after the specified

file. Specifically, in the above scenario, the following files are

present, in chronological order:

    -rw------- 1 torbackup torbackup   16777216 May 10 05:08 main.WAL.00000001000000D9000000AA

    -rw------- 1 torbackup torbackup   16777216 May 10 05:47 main.WAL.00000001000000D9000000AB

    -rw------- 1 torbackup torbackup   16777216 May 10 06:20 main.WAL.00000001000000D9000000AC

    -rw------- 1 torbackup torbackup   16777216 May 10 06:26 main.WAL.00000001000000D9000000AD

    -rw------- 1 torbackup torbackup   16777216 May 10 13:57 main.WAL.00000001000000D9000000B5

Notice the jump from `...AD` to `...B5`. We're missing `AE`, `AF`,

`B1`, `B2`, `B3`, `B4`, specifically. We can also tell that something

happened between 6:26 and 13:57 on that day. It could be that the

backup server went down during that time.

 1. List the files in chronological order:

        ls -ltr /srv/backups/pg/troodi/ | less

 2. Find the file warned about, using `/` then the filename

    (`main.WAL.00000001000000D9000000AD`), above

 3. Look for a `.BASE.` file *following* the missing file, using `/`

    again

 4. Either:

    * if a `.BASE.` backup is present after the missing files, it is

      harmless insofar as the missing timeframe is not

      necessary. TODO: how do we fix the warning anyways?

TODO: there's some information about backup handling in the [Debian

DSA documentation](https://dsa.debian.org/howto/postgres-backup/).

    * if a `.BASE.` backup is *not* present after the missing files,

      the backup integrity is faulty, and a new base backup needs to

      be performed. See [Running a full

      backup](#running-a-full-backup) above.

Reference

=========