source: server/databases/upgradedb.xml@ 7f6bf79b

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../../general.ent">
  %general-entities;
]>

<sect1 id="upgradedb" xreflabel="Upgrade Database">
  <?dbhtml filename="upgradedb.html"?>

  <sect1info>
    <date>$Date$</date>
  </sect1info>

  <title>Important notes about upgrading Database Server Software</title>

    <para>Lets start this chapter a bit dramatic with a screenshot
    of an issue that really happened. Don't be scared if you read
    this the first time, this issue will not occur if you are going
    to install the software the first time:</para>

<screen>$ sudo systemctl status postgresql
-- postgresql.service - PostgreSQL database server
     Loaded: loaded (/usr/lib/systemd/system/postgresql.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Tue 2021-10-26 17:11:53 CDT; 2min 49s ago
    Process: 17336 ExecStart=/usr/bin/pg_ctl -s -D ${PGROOT}/data start -w -t 120 (code=exited, status=1/FAILURE)
        CPU: 7ms

Oct 26 17:11:53 SVRNAME systemd[1]: Starting PostgreSQL database server...
Oct 26 17:11:53 SRVNAME postgres[17338]: 2021-10-26 17:11:53.420 CDT [17338] FATAL:  database files are incompatible with server
Oct 26 17:11:53 SRVNAME postgres[17338]: 2021-10-26 17:11:53.420 CDT [17338] DETAIL:  The data directory was initialized by PostgreSQL version 13, which is not compatible with this version 14.0.
Oct 26 17:11:53 SRVNAME postgres[17336]: pg_ctl: could not start server
Oct 26 17:11:53 SRVNAME postgres[17336]: Examine the log output.
Oct 26 17:11:53 SRVNAME systemd[1]: postgresql.service: Control process exited, code=exited, status=1/FAILURE
Oct 26 17:11:53 SRVNAME systemd[1]: postgresql.service: Failed with result 'exit-code'.
Oct 26 17:11:53 SRVNAME systemd[1]: Failed to start PostgreSQL database server.</screen>

  <para>
    If you like to avoid those situations like the one above finding
    your database server software refusing to start, it is highly
    recommended to read the following thoughts about how to upgrade
    a DBMS (Database Management System) prior to actually doing the
    upgrade.
  </para>
  <para>
    The root cause of the issue shown above was an upgrade
    of the server software to a newer major version but letting the
    data files untouched. The admin was able to recover without any
    loss of data.
  </para>
  <para>
    Even if you are installing a DBMS the initial time, read this
    section too, it might give you a guideline on how to set up backup
    and restore procedures (at least the strategy for building
    them) which are sufficient for your needs and for the safety
    of your data.
  </para>
  
  <sect2>
    <title>Upgrade database server packages</title>

    <para>
      Usually, DB systems work on a bunch of files which holds
      the database metadata and the data itself. Those files
      are highly optimized in their internal structures for use
      by the server software. When upgrading such a server software,
      it can happen that the server software expects a slightly
      different file format than it has been created by previous
      versions. In the best case, the new software can act on the
      old format as well - while not benefitting from newer formats
      which might result in better performance or of other improvements.
      It can also happen that the new server software will reformat
      the data files automatically at the first start.
    </para>

    <para>
      Unfortunatly, the most likly case is that the new server
      software complains about invalid data file formats and exits.
      When this happens and you have overwritten the installed server
      software with the new one, you are kind of lost as you lsot the
      software which can read the data files and the new software is
      unwilling to do so.
    </para>

    <para>
       Changes in data file formats come usually beside major
       version changes. But there is no guaranty. Before upgrading
       the server software, check the documentation if there are
       changes which will require a data file reformat.
    </para>

    <para>
      Of course, if you have databases with content which is not
      easily to rebuild, it is always a good idea of creating backups
      of the database from time to time. When going to upgrade the
      server software, the time is quite good to run another backup.
    </para>

    <sect3>
      <title>Upgrade by backup and restore</title>

      <note>
        <para>
          A backup is meaningless if there is no verified process
          to restore the data from this backup. When running a
          database server, you should not only create backups but you
          should also verify that the process you designed to fullfill
          the restore task is working properly. When you encounter a
          problem with the restore at the time you urgently have to
          rely on the backup data, it is too late - your database are
          in high danger at this moment.
        </para>
      </note>

      <para>
        In general, most (all?) database server software provides
        some basic tools to create backups of your data. Usually,
        the backups created with those tools can be read by newer
        versions of the software (here the restore tool). The other
        way round (older resote tools can work with newer backup data)
        is not defined and you should <emphasis>never</emphasis>
        blindly assume that it would work. It might do, but usually
        it doesn't.
      </para>

      <para>
        The most easiest way to upgrade your database files is to
      </para>

      <itemizedlist>
        <listitem>
          <para>Create a full database backup using the old tools.</para>
          <para>This step creates an offline copy of the database 
          files ready to be used for long term archiving, for desaster
          recovery or just as a preparation for upgrade. This offline
          backups consists of the full one-to-one copy of the current
          database files or a backup of the files from a certain time
          in history plus all journal data (that is Oracle(R)
          terminology, its called "Continuous Archiving" or "write
          ahead log (WAL)" in Postgresql) containing information about
          changes made to the data content. The later are more quickly
          to create if the DB software provides such kind of journal as
          you only have to save the changes which are recorded in those
          journals from the last backup upto now. The amount of data to
          backup is much less than doing a full backup all the time.</para>
          <para>In terms of upgrading a DB server software, it is
          recommended to perform a full backup (which can be used for
          subsequent incremental backups) but if the amount of data is
          too big an incremental backup will be sufficient, too. Which
          strategy is sufficient for you depends on the amount of data
          stored in your DB (is it a few hundret of table rows or is it
          hundrets of Terabyte. A full backup of the later one isn't done
          quickly (and we assume that the underlaying system of such a DB
          might not be a home brewed LFS anyhow).</para>
        </listitem>
        <listitem>
          <para>Upgrade the server software</para>
          <para>In this step, instructions to build the database
          server software are executed just as they are shown in 
          subsequent sections talking about the DBMs like MariaDB or
          Postgresql. This is building the software as
          it is been done as usual in the BLFS book.</para>
        </listitem>
        <listitem>
          <para>Restore the database by using the new tools.</para>
          <para>To restore the data, the tools of the newly installed
          server software is used. During the restore process, the new
          tools will create and/or upgrade the data files in the
          format the software expects them to be.</para>
        </listitem>
      </itemizedlist>

      <para>
        Since you have already a backup procedure in place (and you
        have tested your restore procedure, don't you?), this way might
        be the easiest way to upgrade as you are going to use well-known
        processes to upgrade just as you allways do - at least in terms
        of the backup and restore.
      </para>

    </sect3>

    <sect3>
      <title>Upgrade data file by using system tools</title>

      <para>
        Some database systems (for instance Postgresql) provides
        a tool which can reformat (=upgrade) the existing database
        files to the new format. Since the upgrading tool has to
        be used from the new server software (the old one cannot
        know anything about a new file format), the old software
        might be overwritten due to installation of the new software.
      </para>

      <para>
        In case you have to restore a backup (may be because running
        the upgrade tool failed) you have to reinstall the old version
        to get back the access to your data.
      </para>

      <para>
        Even though those tools might work one the actual database
        files, it is still highly recommended to create a full backup
        before running them. Any case of failure might result in a
        serious damage of the database files.
      </para>

    </sect3>

  </sect2>

  <sect2>
    <title>Notes for specific DBMS</title>
    
    <sect3>
      <title>PostgreSQL</title>
      
      <!-- might add some advanced notes for the DBMS here -->

      <para>Upstream documentation for Backup &amp; Restore:
        <ulink url="https://www.postgresql.org/docs/current/backup.html"/>
      </para>
      
    </sect3>

    <sect3>
      <title>MariaDB</title>
      
      <!-- might add some advanced notes for the DBMS here -->

      <para>Upstream documentation for Backup &amp; Restore:
        <ulink url="https://mariadb.com/kb/en/backup-and-restore-overview/"/>
      </para>
      
    </sect3>

  </sect2>
  
</sect1>