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<chapter xmlns=""
<title>Running NixOS</title>
<para>This chapter describes various aspects of managing a running
NixOS system, such as how to use the <command>systemd</command>
service manager.</para>
<section><title>Service management</title>
<para>In NixOS, all system services are started and monitored using
the systemd program. Systemd is the “init” process of the system
(i.e. PID 1), the parent of all other processes. It manages a set of
so-called “units”, which can be things like system services
(programs), but also mount points, swap files, devices, targets
(groups of units) and more. Units can have complex dependencies; for
instance, one unit can require that another unit must be successfully
started before the first unit can be started. When the system boots,
it starts a unit named <literal></literal>; the
dependencies of this unit cause all system services to be started,
file systems to be mounted, swap files to be activated, and so
<para>The command <command>systemctl</command> is the main way to
interact with <command>systemd</command>. Without any arguments, it
shows the status of active units:
$ systemctl
-.mount loaded active mounted /
swapfile.swap loaded active active /swapfile
sshd.service loaded active running SSH Daemon loaded active active Graphical Interface
<para>You can ask for detailed status information about a unit, for
instance, the PostgreSQL database service:
$ systemctl status postgresql.service
postgresql.service - PostgreSQL Server
Loaded: loaded (/nix/store/pn3q73mvh75gsrl8w7fdlfk3fq5qm5mw-unit/postgresql.service)
Active: active (running) since Mon, 2013-01-07 15:55:57 CET; 9h ago
Main PID: 2390 (postgres)
CGroup: name=systemd:/system/postgresql.service
├─2390 postgres
├─2418 postgres: writer process
├─2419 postgres: wal writer process
├─2420 postgres: autovacuum launcher process
├─2421 postgres: stats collector process
└─2498 postgres: zabbix zabbix [local] idle
Jan 07 15:55:55 hagbard postgres[2394]: [1-1] LOG: database system was shut down at 2013-01-07 15:55:05 CET
Jan 07 15:55:57 hagbard postgres[2390]: [1-1] LOG: database system is ready to accept connections
Jan 07 15:55:57 hagbard postgres[2420]: [1-1] LOG: autovacuum launcher started
Jan 07 15:55:57 hagbard systemd[1]: Started PostgreSQL Server.
Note that this shows the status of the unit (active and running), all
the processes belonging to the service, as well as the most recent log
messages from the service.
<para>Units can be stopped, started or restarted:
$ systemctl stop postgresql.service
$ systemctl start postgresql.service
$ systemctl restart postgresql.service
These operations are synchronous: they wait until the service has
finished starting or stopping (or has failed). Starting a unit will
cause the dependencies of that unit to be started as well (if
<!-- - cgroups: each service and user session is a cgroup
- cgroup resource management -->
<section><title>Rebooting and shutting down</title>
<para>The system can be shut down (and automatically powered off) by
$ shutdown
This is equivalent to running <command>systemctl
<para>To reboot the system, run
$ reboot
which is equivalent to <command>systemctl reboot</command>.
Alternatively, you can quickly reboot the system using
<literal>kexec</literal>, which bypasses the BIOS by directly loading
the new kernel into memory:
$ systemctl kexec
<para>The machine can be suspended to RAM (if supported) using
<command>systemctl suspend</command>, and suspended to disk using
<command>systemctl hibernate</command>.</para>
<para>These commands can be run by any user who is logged in locally,
i.e. on a virtual console or in X11; otherwise, the user is asked for
<section><title>User sessions</title>
<para>Systemd keeps track of all users who are logged into the system
(e.g. on a virtual console or remotely via SSH). The command
<command>loginctl</command> allows querying and manipulating user
sessions. For instance, to list all user sessions:
$ loginctl
c1 500 eelco seat0
c3 0 root seat0
c4 500 alice
This shows that two users are logged in locally, while another is
logged in remotely. (“Seats” are essentially the combinations of
displays and input devices attached to the system; usually, there is
only one seat.) To get information about a session:
$ loginctl session-status c3
c3 - root (0)
Since: Tue, 2013-01-08 01:17:56 CET; 4min 42s ago
Leader: 2536 (login)
Seat: seat0; vc3
TTY: /dev/tty3
Service: login; type tty; class user
State: online
CGroup: name=systemd:/user/root/c3
├─ 2536 /nix/store/10mn4xip9n7y9bxqwnsx7xwx2v2g34xn-shadow- --
├─10339 -bash
└─10355 w3m
This shows that the user is logged in on virtual console 3. It also
lists the processes belonging to this session. Since systemd keeps
track of this, you can terminate a session in a way that ensures that
all the session’s processes are gone:
$ loginctl terminate-session c3
<section><title>Control groups</title>
<para>To keep track of the processes in a running system, systemd uses
<emphasis>control groups</emphasis> (cgroups). A control group is a
set of processes used to allocate resources such as CPU, memory or I/O
bandwidth. There can be multiple control group hierarchies, allowing
each kind of resource to be managed independently.</para>
<para>The command <command>systemd-cgls</command> lists all control
groups in the <literal>systemd</literal> hierarchy, which is what
systemd uses to keep track of the processes belonging to each service
or user session:
$ systemd-cgls
│ └─eelco
│ └─c1
│ ├─ 2567 -:0
│ ├─ 2682 kdeinit4: kdeinit4 Running...
│ ├─ <replaceable>...</replaceable>
│ └─10851 sh -c less -R
│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
│ └─<replaceable>...</replaceable>
│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
└─ <replaceable>...</replaceable>
Similarly, <command>systemd-cgls cpu</command> shows the cgroups in
the CPU hierarchy, which allows per-cgroup CPU scheduling priorities.
By default, every systemd service gets its own CPU cgroup, while all
user sessions are in the top-level CPU cgroup. This ensures, for
instance, that a thousand run-away processes in the
<literal>httpd.service</literal> cgroup cannot starve the CPU for one
process in the <literal>postgresql.service</literal> cgroup. (By
contrast, it they were in the same cgroup, then the PostgreSQL process
would get 1/1001 of the cgroup’s CPU time.) You can limit a service’s
CPU share in <filename>configuration.nix</filename>:
<programlisting> = 512;
By default, every cgroup has 1024 CPU shares, so this will halve the
CPU allocation of the <literal>httpd.service</literal> cgroup.</para>
<para>There also is a <literal>memory</literal> hierarchy that
controls memory allocation limits; by default, all processes are in
the top-level cgroup, so any service or session can exhaust all
available memory. Per-cgroup memory limits can be specified in
<filename>configuration.nix</filename>; for instance, to limit
<literal>httpd.service</literal> to 512 MiB of RAM (excluding swap)
and 640 MiB of RAM (including swap):
<programlisting> = "512M"; = [ "memory.memsw.limit_in_bytes 640M" ];
<para>The command <command>systemd-cgtop</command> shows a
continuously updated list of all cgroups with their CPU and memory
<para>System-wide logging is provided by systemd’s
<emphasis>journal</emphasis>, which subsumes traditional logging
daemons such as syslogd and klogd. Log entries are kept in binary
files in <filename>/var/log/journal/</filename>. The command
<literal>journalctl</literal> allows you to see the contents of the
journal. For example,
$ journalctl -b
shows all journal entries since the last reboot. (The output of
<command>journalctl</command> is piped into <command>less</command> by
default.) You can use various options and match operators to restrict
output to messages of interest. For instance, to get all messages
from PostgreSQL:
$ journalctl -u postgresql.service
-- Logs begin at Mon, 2013-01-07 13:28:01 CET, end at Tue, 2013-01-08 01:09:57 CET. --
Jan 07 15:44:14 hagbard postgres[2681]: [2-1] LOG: database system is shut down
-- Reboot --
Jan 07 15:45:10 hagbard postgres[2532]: [1-1] LOG: database system was shut down at 2013-01-07 15:44:14 CET
Jan 07 15:45:13 hagbard postgres[2500]: [1-1] LOG: database system is ready to accept connections
Or to get all messages since the last reboot that have at least a
“critical” severity level:
$ journalctl -b -p crit
Dec 17 21:08:06 mandark sudo[3673]: pam_unix(sudo:auth): auth could not identify password for [alice]
Dec 29 01:30:22 mandark kernel[6131]: [1053513.909444] CPU6: Core temperature above threshold, cpu clock throttled (total events = 1)
<para>The system journal is readable by root and by users in the
<literal>wheel</literal> and <literal>systemd-journal</literal>
groups. All users have a private journal that can be read using
<section><title>Cleaning up the Nix store</title>
<para>Nix has a purely functional model, meaning that packages are
never upgraded in place. Instead new versions of packages end up in a
different location in the Nix store (<filename>/nix/store</filename>).
You should periodically run Nix’s <emphasis>garbage
collector</emphasis> to remove old, unreferenced packages. This is
$ nix-collect-garbage
Alternatively, you can use a systemd unit that does the same in the
$ systemctl start nix-gc.service
You can tell NixOS in <filename>configuration.nix</filename> to run
this unit automatically at certain points in time, for instance, every
night at 03:15:
nix.gc.automatic = true;
nix.gc.dates = "03:15";
<para>The commands above do not remove garbage collector roots, such
as old system configurations. Thus they do not remove the ability to
roll back to previous configurations. The following command deletes
old roots, removing the ability to roll back to them:
$ nix-collect-garbage -d
You can also do this for specific profiles, e.g.
$ nix-env -p /nix/var/nix/profiles/per-user/eelco/profile --delete-generations old
Note that NixOS system configurations are stored in the profile
<para>Another way to reclaim disk space (often as much as 40% of the
size of the Nix store) is to run Nix’s store optimiser, which seeks
out identical files in the store and replaces them with hard links to
a single copy.
$ nix-store --optimise
Since this command needs to read the entire Nix store, it can take
quite a while to finish.</para>