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><H1
CLASS="SECT1"
><A
NAME="CREATING-CLUSTER"
>18.2. Creating a Database Cluster</A
></H1
><P
>   Before you can do anything, you must initialize a database storage
   area on disk. We call this a <I
CLASS="FIRSTTERM"
>database cluster</I
>.
   (The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> standard uses the term catalog cluster.) A
   database cluster is a collection of databases that is managed by a
   single instance of a running database server. After initialization, a
   database cluster will contain a database named <TT
CLASS="LITERAL"
>postgres</TT
>,
   which is meant as a default database for use by utilities, users and third
   party applications.  The database server itself does not require the
   <TT
CLASS="LITERAL"
>postgres</TT
> database to exist, but many external utility
   programs assume it exists.  Another database created within each cluster
   during initialization is called
   <TT
CLASS="LITERAL"
>template1</TT
>.  As the name suggests, this will be used
   as a template for subsequently created databases; it should not be
   used for actual work.  (See <A
HREF="managing-databases.html"
>Chapter 22</A
> for
   information about creating new databases within a cluster.)
  </P
><P
>   In file system terms, a database cluster is a single directory
   under which all data will be stored. We call this the <I
CLASS="FIRSTTERM"
>data
   directory</I
> or <I
CLASS="FIRSTTERM"
>data area</I
>. It is
   completely up to you where you choose to store your data.  There is no
   default, although locations such as
   <TT
CLASS="FILENAME"
>/usr/local/pgsql/data</TT
> or
   <TT
CLASS="FILENAME"
>/var/lib/pgsql/data</TT
> are popular. To initialize a
   database cluster, use the command <A
HREF="app-initdb.html"
>initdb</A
>, which is
   installed with <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. The desired
   file system location of your database cluster is indicated by the
   <TT
CLASS="OPTION"
>-D</TT
> option, for example:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>initdb -D /usr/local/pgsql/data</KBD
></PRE
><P>
   Note that you must execute this command while logged into the
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> user account, which is
   described in the previous section.
  </P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>    As an alternative to the <TT
CLASS="OPTION"
>-D</TT
> option, you can set
    the environment variable <TT
CLASS="ENVAR"
>PGDATA</TT
>.
    
   </P
></BLOCKQUOTE
></DIV
><P
>   Alternatively, you can run <TT
CLASS="COMMAND"
>initdb</TT
> via
   the <A
HREF="app-pg-ctl.html"
><SPAN
CLASS="APPLICATION"
>pg_ctl</SPAN
></A
>
   program like so:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>pg_ctl -D /usr/local/pgsql/data initdb</KBD
></PRE
><P>
   This may be more intuitive if you are
   using <TT
CLASS="COMMAND"
>pg_ctl</TT
> for starting and stopping the
   server (see <A
HREF="server-start.html"
>Section 18.3</A
>), so
   that <TT
CLASS="COMMAND"
>pg_ctl</TT
> would be the sole command you use
   for managing the database server instance.
  </P
><P
>   <TT
CLASS="COMMAND"
>initdb</TT
> will attempt to create the directory you
   specify if it does not already exist.  Of course, this will fail if
   <TT
CLASS="COMMAND"
>initdb</TT
> does not have permissions to write in the
   parent directory.  It's generally recommendable that the
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> user own not just the data
   directory but its parent directory as well, so that this should not
   be a problem.  If the desired parent directory doesn't exist either,
   you will need to create it first, using root privileges if the
   grandparent directory isn't writable.  So the process might look
   like this:
</P><PRE
CLASS="SCREEN"
>root# <KBD
CLASS="USERINPUT"
>mkdir /usr/local/pgsql</KBD
>
root# <KBD
CLASS="USERINPUT"
>chown postgres /usr/local/pgsql</KBD
>
root# <KBD
CLASS="USERINPUT"
>su postgres</KBD
>
postgres$ <KBD
CLASS="USERINPUT"
>initdb -D /usr/local/pgsql/data</KBD
></PRE
><P>
  </P
><P
>   <TT
CLASS="COMMAND"
>initdb</TT
> will refuse to run if the data directory
   exists and already contains files; this is to prevent accidentally
   overwriting an existing installation.
  </P
><P
>   Because the data directory contains all the data stored in the
   database, it is essential that it be secured from unauthorized
   access. <TT
CLASS="COMMAND"
>initdb</TT
> therefore revokes access
   permissions from everyone but the
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> user.
  </P
><P
>   However, while the directory contents are secure, the default
   client authentication setup allows any local user to connect to the
   database and even become the database superuser. If you do not
   trust other local users, we recommend you use one of
   <TT
CLASS="COMMAND"
>initdb</TT
>'s <TT
CLASS="OPTION"
>-W</TT
>, <TT
CLASS="OPTION"
>--pwprompt</TT
>
   or <TT
CLASS="OPTION"
>--pwfile</TT
> options to assign a password to the
   database superuser.
   Also, specify <TT
CLASS="OPTION"
>-A md5</TT
> or
   <TT
CLASS="OPTION"
>-A password</TT
> so that the default <TT
CLASS="LITERAL"
>trust</TT
> authentication
   mode is not used; or modify the generated <TT
CLASS="FILENAME"
>pg_hba.conf</TT
>
   file after running <TT
CLASS="COMMAND"
>initdb</TT
>, but
   <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>before</I
></SPAN
> you start the server for the first time. (Other
   reasonable approaches include using <TT
CLASS="LITERAL"
>peer</TT
> authentication
   or file system permissions to restrict connections. See <A
HREF="client-authentication.html"
>Chapter 20</A
> for more information.)
  </P
><P
>   <TT
CLASS="COMMAND"
>initdb</TT
> also initializes the default
   locale for the database cluster.
   Normally, it will just take the locale settings in the environment
   and apply them to the initialized database.  It is possible to
   specify a different locale for the database; more information about
   that can be found in <A
HREF="locale.html"
>Section 23.1</A
>.  The default sort order used
   within the particular database cluster is set by
   <TT
CLASS="COMMAND"
>initdb</TT
>, and while you can create new databases using
   different sort order, the order used in the template databases that initdb
   creates cannot be changed without dropping and recreating them.
   There is also a performance impact for using locales
   other than <TT
CLASS="LITERAL"
>C</TT
> or <TT
CLASS="LITERAL"
>POSIX</TT
>. Therefore, it is
   important to make this choice correctly the first time.
  </P
><P
>   <TT
CLASS="COMMAND"
>initdb</TT
> also sets the default character set encoding
   for the database cluster.  Normally this should be chosen to match the
   locale setting.  For details see <A
HREF="multibyte.html"
>Section 23.3</A
>.
  </P
><P
>   Non-<TT
CLASS="LITERAL"
>C</TT
> and non-<TT
CLASS="LITERAL"
>POSIX</TT
> locales rely on the
   operating system's collation library for character set ordering.
   This controls the ordering of keys stored in indexes.  For this reason,
   a cluster cannot switch to an incompatible collation library version,
   either through snapshot restore, binary streaming replication, a
   different operating system, or an operating system upgrade.
  </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="CREATING-CLUSTER-MOUNT-POINTS"
>18.2.1. Use of Secondary File Systems</A
></H2
><P
>    Many installations create their database clusters on file systems
    (volumes) other than the machine's <SPAN
CLASS="QUOTE"
>"root"</SPAN
> volume.  If you
    choose to do this, it is not advisable to try to use the secondary
    volume's topmost directory (mount point) as the data directory.
    Best practice is to create a directory within the mount-point
    directory that is owned by the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
    user, and then create the data directory within that.  This avoids
    permissions problems, particularly for operations such
    as <SPAN
CLASS="APPLICATION"
>pg_upgrade</SPAN
>, and it also ensures clean failures if
    the secondary volume is taken offline.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="CREATING-CLUSTER-NFS"
>18.2.2. Use of Network File Systems</A
></H2
><P
>    Many installations create their database clusters on network file
    systems.  Sometimes this is done via <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
>, or by using a
    Network Attached Storage (<ACRONYM
CLASS="ACRONYM"
>NAS</ACRONYM
>) device that uses
    <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
> internally.  <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does nothing
    special for <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
> file systems, meaning it assumes
    <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
> behaves exactly like locally-connected drives.
    If the client or server <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
> implementation does not
    provide standard file system semantics, this can
    cause reliability problems (see <A
HREF="http://www.time-travellers.org/shane/papers/NFS_considered_harmful.html"
TARGET="_top"
>http://www.time-travellers.org/shane/papers/NFS_considered_harmful.html</A
>).
    Specifically, delayed (asynchronous) writes to the <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
>
    server can cause data corruption problems.  If possible, mount the
    <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
> file system synchronously (without caching) to avoid
    this hazard.  Also, soft-mounting the <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
> file system is
    not recommended.
   </P
><P
>    Storage Area Networks (<ACRONYM
CLASS="ACRONYM"
>SAN</ACRONYM
>) typically use communication
    protocols other than <ACRONYM
CLASS="ACRONYM"
>NFS</ACRONYM
>, and may or may not be subject
    to hazards of this sort.  It's advisable to consult the vendor's
    documentation concerning data consistency guarantees.
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> cannot be more reliable than
    the file system it's using.
   </P
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