18.2. Creating a Database Cluster
Before you can do anything, you must initialize a database storage area on disk. We call this adatabase cluster. (TheSQL 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 namedpostgres, which is meant as a default database for use by utilities, users and third party applications. The database server itself does not require thepostgres database to exist, but many external utility programs assume it exists. Another database created within each cluster during initialization is calledtemplate1. As the name suggests, this will be used as a template for subsequently created databases; it should not be used for actual work. (SeeChapter 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 thedata directory ordata 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 commandinitdb, which is installed withPostgreSQL. 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 thePostgreSQL user account, which is described in the previous section.
Alternatively, you can runinitdb via thepg_ctl program like so:
$pg_ctl -D /usr/local/pgsql/data initdb
This may be more intuitive if you are usingpg_ctl for starting and stopping the server (seeSection 18.3), so thatpg_ctl would be the sole command you use for managing the database server instance.
initdb will attempt to create the directory you specify if it does not already exist. Of course, this will fail ifinitdb does not have permissions to write in the parent directory. It's generally recommendable that thePostgreSQL user own not just the data directory but its parent directory as well, so that this should not be a problem. If the desired parent directory doesn't exist either, you will need to create it first, using root privileges if the grandparent directory isn't writable. So the process might look like this:
root#mkdir /usr/local/pgsqlroot#chown postgres /usr/local/pgsqlroot#su postgrespostgres$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 thePostgreSQL 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 restartingPostgreSQL. Otherwise, a mix of modes might exist in the data directory. For clusters that allow access only by the owner, the appropriate modes are0700 for directories and0600 for files. For clusters that also allow reads by the group, the appropriate modes are0750 for directories and0640 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 ofinitdb's-W,--pwprompt or--pwfile options to assign a password to the database superuser. Also, specify-A md5 or-A password so that the defaulttrust authentication mode is not used; or modify the generatedpg_hba.conf file after runninginitdb, butbefore you start the server for the first time. (Other reasonable approaches include usingpeer authentication or file system permissions to restrict connections. SeeChapter 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 inSection 23.1. The default sort order used within the particular database cluster is set byinitdb, 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 thanC orPOSIX. 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 seeSection 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.
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 thePostgreSQL user, and then create the data directory within that. This avoids permissions problems, particularly for operations such aspg_upgrade, and it also ensures clean failures if the secondary volume is taken offline.
18.2.2. File Systems
Generally, any file system with POSIX semantics can be used for PostgreSQL. Users prefer different file systems for a variety of reasons, including vendor support, performance, and familiarity. Experience suggests that, all other things being equal, one should not expect major performance or behavior changes merely from switching file systems or making minor file system configuration changes.
18.2.2.1. NFS
It is possible to use anNFS file system for storing thePostgreSQL data directory.PostgreSQL does nothing special forNFS file systems, meaning it assumesNFS behaves exactly like locally-connected drives.PostgreSQL does not use any functionality that is known to have nonstandard behavior onNFS, such as file locking. The only firm requirement for usingNFS withPostgreSQL is that the file system is mounted using the It is not necessary to use the In some cases, an external storage product can be accessed either via NFS or a lower-level protocol such as iSCSI. In the latter case, the storage appears as a block device and any available file system can be created on it. That approach might relieve the DBA from having to deal with some of the idiosyncrasies of NFS, but of course the complexity of managing remote storage then happens at other levels.hard option. With thehard option, processes can“hang” indefinitely if there are network problems, so this configuration will require a careful monitoring setup. Thesoft option will interrupt system calls in case of network problems, butPostgreSQL will not repeat system calls interrupted in this way, so any such interruption will result in an I/O error being reported.sync mount option. The behavior of theasync option is sufficient, sincePostgreSQL issuesfsync calls at appropriate times to flush the write caches. (This is analogous to how it works on a local file system.) However, it is strongly recommended to use thesync export option on the NFSserver on systems where it exists (mainly Linux). Otherwise, anfsync or equivalent on the NFS client is not actually guaranteed to reach permanent storage on the server, which could cause corruption similar to running with the parameterfsync off. The defaults of these mount and export options differ between vendors and versions, so it is recommended to check and perhaps specify them explicitly in any case to avoid any ambiguity.