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><H1
CLASS="SECT1"
><A
NAME="CREATING-CLUSTER"
>17.2. 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 21</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 17.3</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 19</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 22.1</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 22.3</A
>.
</P
><P
> Non-<TT
CLASS="LITERAL"
>C</TT
> and 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"
>17.2.1. 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"
>17.2.2. 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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