контейнеры linux (linux containers)
FUNCTIONAL SPECIFICATION
A container is an object isolating some resources of the host,
for the application or system running in it.
The application / system will be launched inside a container
specified by a configuration that is either initially created or
passed as a parameter of the commands.
How to run an application in a container
Before running an application, you should know what are the
resources you want to isolate. The default configuration is to
isolate PIDs, the sysv IPC and mount points. If you want to run a
simple shell inside a container, a basic configuration is needed,
especially if you want to share the rootfs. If you want to run an
application like sshd, you should provide a new network stack and
a new hostname. If you want to avoid conflicts with some files
eg. /var/run/httpd.pid, you should remount /var/run with an
empty directory. If you want to avoid the conflicts in all the
cases, you can specify a rootfs for the container. The rootfs can
be a directory tree, previously bind mounted with the initial
rootfs, so you can still use your distro but with your own /etc
and /home
Here is an example of directory tree for sshd:
[root@lxc sshd]$ tree -d rootfs
rootfs
|-- bin
|-- dev
| |-- pts
| `-- shm
| `-- network
|-- etc
| `-- ssh
|-- lib
|-- proc
|-- root
|-- sbin
|-- sys
|-- usr
`-- var
|-- empty
| `-- sshd
|-- lib
| `-- empty
| `-- sshd
`-- run
`-- sshd
and the mount points file associated with it:
[root@lxc sshd]$ cat fstab
/lib /home/root/sshd/rootfs/lib none ro,bind 0 0
/bin /home/root/sshd/rootfs/bin none ro,bind 0 0
/usr /home/root/sshd/rootfs/usr none ro,bind 0 0
/sbin /home/root/sshd/rootfs/sbin none ro,bind 0 0
How to run a system in a container
Running a system inside a container is paradoxically easier than
running an application. Why? Because you don't have to care about
the resources to be isolated, everything needs to be isolated,
the other resources are specified as being isolated but without
configuration because the container will set them up. eg. the
ipv4 address will be setup by the system container init scripts.
Here is an example of the mount points file:
[root@lxc debian]$ cat fstab
/dev /home/root/debian/rootfs/dev none bind 0 0
/dev/pts /home/root/debian/rootfs/dev/pts none bind 0 0
CONTAINER LIFE CYCLE
When the container is created, it contains the configuration
information. When a process is launched, the container will be
starting and running. When the last process running inside the
container exits, the container is stopped.
In case of failure when the container is initialized, it will
pass through the aborting state.
---------
| STOPPED |<---------------
--------- |
| |
start |
| |
V |
---------- |
| STARTING |--error- |
---------- | |
| | |
V V |
--------- ---------- |
| RUNNING | | ABORTING | |
--------- ---------- |
| | |
no process | |
| | |
V | |
---------- | |
| STOPPING |<------- |
---------- |
| |
---------------------
CONFIGURATION
The container is configured through a configuration file, the
format of the configuration file is described in lxc.conf(5)
CREATING / DESTROYING CONTAINERS
A persistent container object can be created via the lxc-create
command. It takes a container name as parameter and optional
configuration file and template. The name is used by the
different commands to refer to this container. The lxc-destroy
command will destroy the container object.
lxc-create -n foo
lxc-destroy -n foo
VOLATILE CONTAINER
It is not mandatory to create a container object before starting
it. The container can be directly started with a configuration
file as parameter.
STARTING / STOPPING CONTAINER
When the container has been created, it is ready to run an
application / system. This is the purpose of the lxc-execute and
lxc-start commands. If the container was not created before
starting the application, the container will use the
configuration file passed as parameter to the command, and if
there is no such parameter either, then it will use a default
isolation. If the application ended, the container will be
stopped, but if needed the lxc-stop command can be used to stop
the container.
Running an application inside a container is not exactly the same
thing as running a system. For this reason, there are two
different commands to run an application into a container:
lxc-execute -n foo [-f config] /bin/bash
lxc-start -n foo [-f config] [/bin/bash]
The lxc-execute command will run the specified command into a
container via an intermediate process, lxc-init. This lxc-init
after launching the specified command, will wait for its end and
all other reparented processes. (to support daemons in the
container). In other words, in the container, lxc-init has PID 1
and the first process of the application has PID 2.
The lxc-start command will directly run the specified command in
the container. The PID of the first process is 1. If no command
is specified lxc-start will run the command defined in
lxc.init.cmd or if not set, /sbin/init.
To summarize, lxc-execute is for running an application and lxc-
start is better suited for running a system.
If the application is no longer responding, is inaccessible or is
not able to finish by itself, a wild lxc-stop command will kill
all the processes in the container without pity.
lxc-stop -n foo -k
CONNECT TO AN AVAILABLE TTY
If the container is configured with ttys, it is possible to
access it through them. It is up to the container to provide a
set of available ttys to be used by the following command. When
the tty is lost, it is possible to reconnect to it without login
again.
lxc-console -n foo -t 3
FREEZE / UNFREEZE CONTAINER
Sometime, it is useful to stop all the processes belonging to a
container, eg. for job scheduling. The commands:
lxc-freeze -n foo
will put all the processes in an uninteruptible state and
lxc-unfreeze -n foo
will resume them.
This feature is enabled if the freezer cgroup v1 controller is
enabled in the kernel.
GETTING INFORMATION ABOUT CONTAINER
When there are a lot of containers, it is hard to follow what has
been created or destroyed, what is running or what are the PIDs
running in a specific container. For this reason, the following
commands may be useful:
lxc-ls -f
lxc-info -n foo
lxc-ls lists containers.
lxc-info gives information for a specific container.
Here is an example on how the combination of these commands
allows one to list all the containers and retrieve their state.
for i in $(lxc-ls -1); do
lxc-info -n $i
done
MONITORING CONTAINER
It is sometime useful to track the states of a container, for
example to monitor it or just to wait for a specific state in a
script.
lxc-monitor command will monitor one or several containers. The
parameter of this command accepts a regular expression for
example:
lxc-monitor -n "foo|bar"
will monitor the states of containers named 'foo' and 'bar', and:
lxc-monitor -n ".*"
will monitor all the containers.
For a container 'foo' starting, doing some work and exiting, the
output will be in the form:
'foo' changed state to [STARTING]
'foo' changed state to [RUNNING]
'foo' changed state to [STOPPING]
'foo' changed state to [STOPPED]
lxc-wait command will wait for a specific state change and exit.
This is useful for scripting to synchronize the launch of a
container or the end. The parameter is an ORed combination of
different states. The following example shows how to wait for a
container if it successfully started as a daemon.
# launch lxc-wait in background
lxc-wait -n foo -s STOPPED &
LXC_WAIT_PID=$!
# this command goes in background
lxc-execute -n foo mydaemon &
# block until the lxc-wait exits
# and lxc-wait exits when the container
# is STOPPED
wait $LXC_WAIT_PID
echo "'foo' is finished"
CGROUP SETTINGS FOR CONTAINERS
The container is tied with the control groups, when a container
is started a control group is created and associated with it. The
control group properties can be read and modified when the
container is running by using the lxc-cgroup command.
lxc-cgroup command is used to set or get a control group
subsystem which is associated with a container. The subsystem
name is handled by the user, the command won't do any syntax
checking on the subsystem name, if the subsystem name does not
exists, the command will fail.
lxc-cgroup -n foo cpuset.cpus
will display the content of this subsystem.
lxc-cgroup -n foo cpu.shares 512
will set the subsystem to the specified value.
