18.2. Creating a Database Cluster
Before you can do anything, you must initialize a database storage
area on disk. We call this a database cluster.
(The SQL 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
which is meant as a default database for use by utilities, users and third
party applications. The database server itself does not require the
postgres database to exist, but many external utility
programs assume it exists. Another database created within each cluster
during initialization is called
template1. As the name suggests, this will be used
as a template for subsequently created databases; it should not be
used for actual work. (See Chapter 22 for
information about creating new databases within a cluster.)
In file system terms, a database cluster is a single directory
under which all data will be stored. We call this the data
directory or data area. It is
completely up to you where you choose to store your data. There is no
default, although locations such as
/var/lib/pgsql/data are popular. To initialize a
database cluster, use the command initdb, which is
installed with PostgreSQL. The desired
file system location of your database cluster is indicated by the
-D option, for example:
initdb -D /usr/local/pgsql/data
Note that you must execute this command while logged into the PostgreSQL user account, which is described in the previous section.
Alternatively, you can run
program like so:
pg_ctl -D /usr/local/pgsql/data initdb
This may be more intuitive if you are
pg_ctl for starting and stopping the
server (see Section 18.3), so
pg_ctl would be the sole command you use
for managing the database server instance.
initdb will attempt to create the directory you
specify if it does not already exist. Of course, this will fail if
initdb does not have permissions to write in the
parent directory. It's generally recommendable that the
PostgreSQL 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
chown postgres /usr/local/pgsqlroot#
initdb -D /usr/local/pgsql/data
initdb will refuse to run if the data directory
exists and already contains files; this is to prevent accidentally
overwriting an existing installation.
Because the data directory contains all the data stored in the
database, it is essential that it be secured from unauthorized
initdb therefore revokes access
permissions from everyone but the
PostgreSQL user, and optionally, group.
Group access, when enabled, is read-only. This allows an unprivileged
user in the same group as the cluster owner to take a backup of the
cluster data or perform other operations that only require read access.
Note that enabling or disabling group access on an existing cluster requires
the cluster to be shut down and the appropriate mode to be set on all
directories and files before restarting
PostgreSQL. Otherwise, a mix of modes might
exist in the data directory. For clusters that allow access only by the
owner, the appropriate modes are
0700 for directories
0600 for files. For clusters that also allow
reads by the group, the appropriate modes are
for directories and
0640 for files.
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
--pwfile options to assign a password to the
-A md5 or
-A password so that the default
mode is not used; or modify the generated
file after running
before you start the server for the first time. (Other
reasonable approaches include using
or file system permissions to restrict connections. See Chapter 20 for more information.)
initdb 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 Section 23.1. The default sort order used
within the particular database cluster is set by
initdb, 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
POSIX. Therefore, it is
important to make this choice correctly the first time.
initdb also sets the default character set encoding
for the database cluster. Normally this should be chosen to match the
locale setting. For details see Section 23.3.
C and non-
POSIX 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.
18.2.1. Use of Secondary File Systems
Many installations create their database clusters on file systems (volumes) other than the machine's “root” 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 PostgreSQL user, and then create the data directory within that. This avoids permissions problems, particularly for operations such as pg_upgrade, and it also ensures clean failures if the secondary volume is taken offline.
18.2.2. Use of Network File Systems
Many installations create their database clusters on network file systems. Sometimes this is done via NFS, or by using a Network Attached Storage (NAS) device that uses NFS internally. PostgreSQL does nothing special for NFS file systems, meaning it assumes NFS behaves exactly like locally-connected drives. If the client or server NFS implementation does not provide standard file system semantics, this can cause reliability problems (see https://www.time-travellers.org/shane/papers/NFS_considered_harmful.html). Specifically, delayed (asynchronous) writes to the NFS server can cause data corruption problems. If possible, mount the NFS file system synchronously (without caching) to avoid this hazard. Also, soft-mounting the NFS file system is not recommended.
Storage Area Networks (SAN) typically use communication protocols other than NFS, 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. PostgreSQL cannot be more reliable than the file system it's using.