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 postgres,
   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
   /usr/local/pgsql/data or
   /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 initdb via
   the pg_ctl
   program like so:
$pg_ctl -D /usr/local/pgsql/data initdb
   This may be more intuitive if you are
   using pg_ctl for starting and stopping the
   server (see Section 18.3), so
   that 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
   like this:
root#mkdir /usr/local/pgsqlroot#chown postgres /usr/local/pgsqlroot#su postgrespostgres$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
   access. 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
   and 0600 for files.  For clusters that also allow
   reads by the group, the appropriate modes are 0750
   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
   initdb's -W, --pwprompt
   or --pwfile options to assign a password to the
   database superuser.
   Also, specify -A md5 or
   -A password so that the default trust authentication
   mode is not used; or modify the generated pg_hba.conf
   file after running initdb, but
   before you start the server for the first time. (Other
   reasonable approaches include using peer authentication
   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
   other than C or 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.
  
   Non-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.
  
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.
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.