of Lennart Poettering's "systemd for Administrators" series
is available. "Most services on Linux/Unix are singleton services: there's usually only one instance of Syslog, Postfix, or Apache running on a specific system at the same time. On the other hand some select services may run in multiple instances on the same host. For example, an Internet service like the Dovecot IMAP service could run in multiple instances on different IP ports or different local IP addresses. A more common example that exists on all installations is getty, the mini service that runs once for each TTY and presents a login prompt on it. On most systems this service is instantiated once for each of the first six virtual consoles tty1 to tty6. On some servers depending on administrator configuration or boot-time parameters an additional getty is instantiated for a serial or virtualizer console. Another common instantiated service in the systemd world is fsck, the file system checker that is instantiated once for each block device that needs to be checked. Finally, in systemd socket activated per-connection services (think classic inetd!) are also implemented via instantiated services: a new instance is created for each incoming connection. In this installment I hope to explain a bit how systemd implements instantiated services and how to take advantage of them as an administrator.
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