From Fedora Project Wiki

systemd es un sistema y administrador de servicios para Linux, compatible con scripts de inicio (init) SysV y LSB. systemd proporciona capacidades de paralelización agresivas, utiliza socket y activación D-Bus para iniciar los servicios, ofrece la puesta en marcha de demonios bajo demanda, realiza el seguimiento de procesos utilizando Linux cgroups, soporta copia instantánea de volumen y la restauración de estado del sistema, mantiene puntos de montaje y automontaje e implementa un elaborado servicio lógico de control transaccional basado en la dependencia. Puede funcionar como un reemplazo para sysvinit. Para más información, ver el video en http://linuxconfau.blip.tv/file/4696791/ o http://www.youtube.com/watch?v=TyMLi8QF6sw

Para administradores de sistemas
Los administradores de sistemas pueden visitar esta página, para entender cómo usar las llamadas systemctl nativas que reemplacen su antiguo flujo de trabajo en SysVinit. Tenga en cuenta que los comandos service y chkconfig seguirán trabajando como se esperaba en el mundo de systemd.

¿Por qué systemd?

http://0pointer.de/blog/projects/why.html

Introducción

systemd inicia y supervisa todo el sistema y se basa en la noción de unidades compuestas de un nombre, tipo y coincidencia de un archivo de configuración con el mismo nombre y tipo (por ejemplo, una unidad avahi.service tiene un archivo de configuración con el mismo nombre y es una unidad de encapsulado del demonio Avahi). Existen siete tipos diferentes de unidades:

  1. service: este es el tipo más obvio de unidad: demonios que pueden ser iniciados, detenidos, reiniciados, recargados.
  2. socket: Esta unidad encapsula un socket en el sistema de archivos o en Internet. Actualmente systemd soporta el funcionamiento de los tipos de sockets AF_INET, AF_INET6, AF_UNIX, datagram y paquetes secuenciales. También puede soportar FIFOs clásicos como transporte. Cada unidad socket tiene una unidad de servicio correspondiente, que se inicia si la primera conexión entra en el socket o FIFO (por ejemplo, nscd.socket inicia nscd.service en una conexión entrante).
  3. device: esta unidad encapsula un dispositivo en el árbol de dispositivos de Linux. Si un dispositivo está marcado para ello a través de reglas udev, se expondrá como una unidad device en systemd. Las propiedades establecidas con udev pueden utilizarse como configuración fuente para establecer dependencias para unidades device.
  4. mount: esta unidad encapsula un punto de montaje en la jerarquía del sistema de archivos.
  5. automount: este tipo de unidad encapsula un punto de montaje automático en la jerarquía del sistema de archivos. Cada unidad automount tiene una unidad mount correspondiente, que se inicia (es decir, montada) tan pronto como se accede al directorio de automontaje.
  6. target: este tipo de unidad se utiliza para la agrupación lógica de unidades: en vez de realmente hacer nada por sí misma simplemente hace referencia a otras unidades, que así pueden ser controladas conjuntamente, (por ejemplo, multi-user.target, que es un objetivo que básicamente desempeña el papel de nivel de ejecución 5 en el sistema clásico SysV; o bluetooth.target que es solicitado tan pronto como esté disponible un adaptador bluetooth y que simplemente carga servicios relacionados con bluetooth que de lo contrario no tendrían que iniciarse: bluetoothd, obexd y cosas por el estilo).
  7. snapshot: similar a las unidades target, snapshots en realidad no hacen nada ellas mismas y su único propósito es hacer referencia a otras unidades.

Características de systemd

systemd ofrece lo siguiente:

  • aggressive parallelization capabilities using socket: To speed up the entire boot and start more processes in parallel, systemd creates the listening sockets before actually starting the daemon, and then just passes the socket to it. All sockets for all daemons are created in one step in the init system, and then in a second step run all daemons at once. If a service needs another, and it is not fully started up, what will happen is that the connection is queued in the providing service and the client will potentially block on that single request. But only that one client will block and only on that one request. Also, dependencies between services no longer have to be configured to allow proper parallelized start-up: starting all sockets at once and a service needing another, it surely can connect to its socket.
  • D-Bus activation for starting services: Using bus activation, a service can be started the first time it is accessed. Bus activation also gives the minimal per-request synchronisation needed for starting up the providers and the consumers of D-Bus services at the same time: starting a service at the same time as another, if one is quicker, than via the bus activation logic the D-Bus queues the request until the other manages to establish its service name.
  • offers on-demand starting of daemons
  • keeps track of processes using Linux cgroups: Every executed process gets its own cgroup and it is very easy to configure systemd to place services in cgroups that have been configured externally, for example via the libcgroups utilities.
  • supports snapshotting and restoring of the system state: Snapshots can be used to save/rollback the state of all services and units of the init system. Primarily it has two intended use cases: to allow the user to temporarily enter a specific state such as "Emergency Shell", terminating current services, and provide an easy way to return to the state before, pulling up all services again that got temporarily pulled down.
  • maintains mount and automount points: Systemd monitors all mount points how they come and go, and can also be used to mount or unmount mount-points. /etc/fstab can be used here as an additional configuration source for these mount points. Using the comment= fstab option you can even mark /etc/fstab entries to become systemd controlled automount points..
  • implements an elaborate transactional dependency-based service control logic: Systemd supports several kinds of dependencies between services (or units), using After/Before, Requires and Wants options in the unit configuration files to fix the ordering how units are activated. Requires and Wants, express a positive requirement dependency, either mandatory, or optional. There is Conflicts which expresses a negative requirement dependency, and others less used. As a transactional control, if a unit is requested to start up or shut down, systemd will add it and all its dependencies to a temporary transaction, verifing if the transaction is consistent (or the ordering via After/Before of all units is cycle-free). If it is not, systemd will try to fix it up, and removes non-essential jobs from the transaction that might remove the loop.

and:

  • For each process spawned, it controls: The environment, resource limits, working and root directory, umask, OOM killer adjustment, nice level, IO class and priority, CPU policy and priority, CPU affinity, timer slack, user id, group id, supplementary group ids, readable/writable/inaccessible directories, shared/private/slave mount flags, capabilities/bounding set, secure bits, CPU scheduler reset of fork, private /tmp name-space, cgroup control for various subsystems. Also, you can easily connect stdin/stdout/stderr of services to syslog, /dev/kmsg, arbitrary TTYs. If connected to a TTY for input systemd will make sure a process gets exclusive access, optionally waiting or enforcing it.
  • The native configuration files use a syntax that closely follows the well-known .desktop files: It is a simple syntax for which parsers exist already in many software frameworks. Also, this allows to rely on existing tools for i18n for service descriptions, and similar, without for admins the need to learn a new syntax.
systemadm
There's a minimal UI, systemadm that allows to start/stop/introspect services. It is part of the systemd-gtk package It's a work in progress and is not functional yet, but useful as a debugging tool. It's written in Vala. Do not use it for now unless you are a developer

(... and more advanced)

  • Compatibility with SysV init scripts: It takes advantages of LSB and Red Hat chkconfig headers if they are available, if not, it uses otherwise available information, such as the start priorities in /etc/rc.d. These init scripts are simply considered a different source of configuration, easing the upgrade path to proper systemd services.
  • /etc/fstab configuration file: it just another source of configuration. Using the comment= fstab option it is possible to mark /etc/fstab entries to become systemd controlled automount points.
  • Support a simple templating/instance mechanism: For example, instead of having six configuration files for six gettys, it has only one getty@.service file which gets instantiated to getty@tty2.service and suchlike. The interface part can even be inherited by dependency expressions, i.e. it is easy to encode that a service dhcpcd@eth0.service pulls in avahi-autoipd@eth0.service, while leaving the eth0 string wild-carded.
  • Compatibility, to a certain extent, with /dev/initctl. This compatibility is in fact implemented with a FIFO-activated service, which simply translates these legacy requests to D-Bus requests. Effectively this means the old shutdown, poweroff and similar commands from Upstart and sysvinit continue to work with systemd.
  • Compatibility with utmp and wtmp (To an extent that is far more than healthy, given how crufty utmp and wtmp today are).

For all details view A short list of other features on the developer blog.

Documentación de systemd

systemd tiene documentación muy completa. Consulte

http://0pointer.de/blog/projects/systemd-docs.html

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