Before you can do anything, you must initialize a database storage area on disk. We call this a database instance. (The SQL standard uses the term catalog cluster.) A database instance is a collection of databases that is managed by a single instance of a running database server. After initialization, a database instance 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 21 for information about creating new databases within a cluster.)

In file system terms, a database instance 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/lightdb-x/data or /var/lib/lightdb-x/data are popular. The data directory must be initialized before being used, using the program lt_initdb which is installed with LightDB.

If you are using a pre-packaged version of LightDB, it may well have a specific convention for where to place the data directory, and it may also provide a script for creating the data directory. In that case you should use that script in preference to running lt_initdb directly. Consult the package-level documentation for details.

To initialize a database instance manually, run lt_initdb and specify the desired file system location of the database instance with the -D option, for example:

$ lt_initdb -D /usr/local/lightdb-x/data

Note that you must execute this command while logged into the LightDB user account, which is described in the previous section.

Alternatively, you can run lt_initdb via the lt_ctl program like so:

$ lt_ctl -D /usr/local/lightdb-x/data lt_initdb

This may be more intuitive if you are using lt_ctl for starting and stopping the server (see Section 17.3), so that lt_ctl would be the sole command you use for managing the database server instance.

lt_initdb will attempt to create the directory you specify if it does not already exist. Of course, this will fail if lt_initdb does not have permissions to write in the parent directory. It's generally recommendable that the LightDB 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/pgsql
root# chown lightdb /usr/local/pgsql
root# su lightdb
lightdb$ lt_initdb -D /usr/local/lightdb-x/data

lt_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. lt_initdb therefore revokes access permissions from everyone but the LightDB 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 LightDB. 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 lt_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 lt_hba.conf file after running lt_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 19 for more information.)

lt_initdb also initializes the default locale for the database instance. 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 22.1. The default sort order used within the particular database instance is set by lt_initdb, and while you can create new databases using different sort order, the order used in the template databases that lt_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.

lt_initdb also sets the default character set encoding for the database instance. Normally this should be chosen to match the locale setting. For details see Section 22.3.

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.