369 lines
13 KiB
XML
369 lines
13 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
|
||
xmlns:xlink="http://www.w3.org/1999/xlink"
|
||
xml:id="ch-running">
|
||
|
||
<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 xml:id="sec-systemctl"><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>default.target</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
|
||
on.</para>
|
||
|
||
<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:
|
||
|
||
<screen>
|
||
$ systemctl
|
||
-.mount loaded active mounted /
|
||
swapfile.swap loaded active active /swapfile
|
||
sshd.service loaded active running SSH Daemon
|
||
graphical.target loaded active active Graphical Interface
|
||
<replaceable>...</replaceable>
|
||
</screen>
|
||
|
||
</para>
|
||
|
||
<para>You can ask for detailed status information about a unit, for
|
||
instance, the PostgreSQL database service:
|
||
|
||
<screen>
|
||
$ 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.
|
||
</screen>
|
||
|
||
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>
|
||
|
||
<para>Units can be stopped, started or restarted:
|
||
|
||
<screen>
|
||
$ systemctl stop postgresql.service
|
||
$ systemctl start postgresql.service
|
||
$ systemctl restart postgresql.service
|
||
</screen>
|
||
|
||
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
|
||
necessary).</para>
|
||
|
||
<!-- - cgroups: each service and user session is a cgroup
|
||
|
||
- cgroup resource management -->
|
||
|
||
</section>
|
||
|
||
|
||
<!--===============================================================-->
|
||
|
||
<section xml:id="sec-rebooting"><title>Rebooting and shutting down</title>
|
||
|
||
<para>The system can be shut down (and automatically powered off) by
|
||
doing:
|
||
|
||
<screen>
|
||
$ shutdown
|
||
</screen>
|
||
|
||
This is equivalent to running <command>systemctl
|
||
poweroff</command>.</para>
|
||
|
||
<para>To reboot the system, run
|
||
|
||
<screen>
|
||
$ reboot
|
||
</screen>
|
||
|
||
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:
|
||
|
||
<screen>
|
||
$ systemctl kexec
|
||
</screen>
|
||
|
||
</para>
|
||
|
||
<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
|
||
authentication.</para>
|
||
|
||
</section>
|
||
|
||
|
||
<!--===============================================================-->
|
||
|
||
<section xml:id="sec-user-sessions"><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:
|
||
|
||
<screen>
|
||
$ loginctl
|
||
SESSION UID USER SEAT
|
||
c1 500 eelco seat0
|
||
c3 0 root seat0
|
||
c4 500 alice
|
||
</screen>
|
||
|
||
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:
|
||
|
||
<screen>
|
||
$ 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-4.1.5.1/bin/login --
|
||
├─10339 -bash
|
||
└─10355 w3m nixos.org
|
||
</screen>
|
||
|
||
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:
|
||
|
||
<screen>
|
||
$ loginctl terminate-session c3
|
||
</screen>
|
||
|
||
</para>
|
||
|
||
</section>
|
||
|
||
|
||
<!--===============================================================-->
|
||
|
||
<section xml:id="sec-cgroups"><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:
|
||
|
||
<screen>
|
||
$ systemd-cgls
|
||
├─user
|
||
│ └─eelco
|
||
│ └─c1
|
||
│ ├─ 2567 -:0
|
||
│ ├─ 2682 kdeinit4: kdeinit4 Running...
|
||
│ ├─ <replaceable>...</replaceable>
|
||
│ └─10851 sh -c less -R
|
||
└─system
|
||
├─httpd.service
|
||
│ ├─2444 httpd -f /nix/store/3pyacby5cpr55a03qwbnndizpciwq161-httpd.conf -DNO_DETACH
|
||
│ └─<replaceable>...</replaceable>
|
||
├─dhcpcd.service
|
||
│ └─2376 dhcpcd --config /nix/store/f8dif8dsi2yaa70n03xir8r653776ka6-dhcpcd.conf
|
||
└─ <replaceable>...</replaceable>
|
||
</screen>
|
||
|
||
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>
|
||
systemd.services.httpd.serviceConfig.CPUShares = 512;
|
||
</programlisting>
|
||
|
||
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>
|
||
systemd.services.httpd.serviceConfig.MemoryLimit = "512M";
|
||
systemd.services.httpd.serviceConfig.ControlGroupAttribute = [ "memory.memsw.limit_in_bytes 640M" ];
|
||
</programlisting>
|
||
|
||
</para>
|
||
|
||
<para>The command <command>systemd-cgtop</command> shows a
|
||
continuously updated list of all cgroups with their CPU and memory
|
||
usage.</para>
|
||
|
||
</section>
|
||
|
||
|
||
<!--===============================================================-->
|
||
|
||
<section xml:id="sec-logging"><title>Logging</title>
|
||
|
||
<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,
|
||
|
||
<screen>
|
||
$ journalctl -b
|
||
</screen>
|
||
|
||
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:
|
||
|
||
<screen>
|
||
$ 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
|
||
</screen>
|
||
|
||
Or to get all messages since the last reboot that have at least a
|
||
“critical” severity level:
|
||
|
||
<screen>
|
||
$ 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)
|
||
</screen>
|
||
|
||
</para>
|
||
|
||
<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
|
||
<command>journalctl</command>.</para>
|
||
|
||
</section>
|
||
|
||
|
||
<!--===============================================================-->
|
||
|
||
<section xml:id="sec-nix-gc"><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
|
||
easy:
|
||
|
||
<screen>
|
||
$ nix-collect-garbage
|
||
</screen>
|
||
|
||
Alternatively, you can use a systemd unit that does the same in the
|
||
background:
|
||
|
||
<screen>
|
||
$ systemctl start nix-gc.service
|
||
</screen>
|
||
|
||
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:
|
||
|
||
<programlisting>
|
||
nix.gc.automatic = true;
|
||
nix.gc.dates = "03:15";
|
||
</programlisting>
|
||
|
||
</para>
|
||
|
||
<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:
|
||
<screen>
|
||
$ nix-collect-garbage -d
|
||
</screen>
|
||
You can also do this for specific profiles, e.g.
|
||
<screen>
|
||
$ nix-env -p /nix/var/nix/profiles/per-user/eelco/profile --delete-generations old
|
||
</screen>
|
||
Note that NixOS system configurations are stored in the profile
|
||
<filename>/nix/var/nix/profiles/system</filename>.</para>
|
||
|
||
<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.
|
||
<screen>
|
||
$ nix-store --optimise
|
||
</screen>
|
||
Since this command needs to read the entire Nix store, it can take
|
||
quite a while to finish.</para>
|
||
|
||
</section>
|
||
|
||
|
||
</chapter>
|