Difference between revisions of "LXC"

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(Example)
(Dump and restore)
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We also find it useful to use the <code>--restore-detached</code> option for restore to make contianer reparent to init rather than hanging on a crtools process launched from shell. Another useful option is the <code>--pidfile</code> one -- you will be able to find out the host-side pid of a container init after restore.
 
We also find it useful to use the <code>--restore-detached</code> option for restore to make contianer reparent to init rather than hanging on a crtools process launched from shell. Another useful option is the <code>--pidfile</code> one -- you will be able to find out the host-side pid of a container init after restore.
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More details on the option mentioned can be found in [[Usage]] and [[Advanced usage]] pages.
  
 
== Example ==
 
== Example ==

Revision as of 13:45, 18 September 2012

Prepare a Linux Container

Requirements

  • A console should be disabled (lxc.console = none)
  • udev should not run inside containers ($ mv /sbin/udevd{,.bcp})

Preparing a host environment

  • Mount cgroupfs
$ mount -t cgroup c /cgroup
  • Create a network bridge
# cat /etc/sysconfig/network-scripts/ifcfg-br0 
DEVICE=br0
TYPE=Bridge
BOOTPROTO=dhcp
ONBOOT=yes
DELAY=5
NM_CONTROLLED=n
$ cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
NM_CONTROLLED="no"
ONBOOT="yes"
BRIDGE=br0

Create and start a container

  • Download an OpenVZ template and extract it.
curl http://download.openvz.org/template/precreated/centos-6-x86_64.tar.gz | tar -xz -C test-lxc
  • Create config files
$ cat ~/test-lxc.conf 
lxc.console=none
lxc.utsname = test-lxc
lxc.network.type = veth
lxc.network.flags = up
lxc.network.link = br0
lxc.network.name = eth0
lxc.mount = /root/test-lxc/etc/fstab
lxc.rootfs = /root/test-lxc-root/
$ cat /root/test-lxc/etc/fstab
none /root/test-lxc-root/dev/pts devpts defaults 0 0
none /root/test-lxc-root/proc    proc   defaults 0 0
none /root/test-lxc-root/sys     sysfs  defaults 0 0
none /root/test-lxc-root/dev/shm tmpfs  defaults 0 0
  • Register the container
$ lxc-create -n test-lxc -f test-lxc.conf
  • Start the container
$ mount --bind test-lxc test-lxc-root/
$ lxc-start -n test-lxc

Checkpoint and restore an LXC Container

Preparations

You only need to install the crtools.

Dump and restore

Dumping and restoring an LXC contianer means -- dumping a subtree of processes starting from container init plus all kinds of namespaces. Restoring is symmetrical. The way LXC container works imposes some more requirements on crtools usage.

  • You need to use the --evasive-devices option to handle /dev/log users (there's a bug in LXC code)
  • In order to properly isolate container from unwanted networking communication during checkpoint/restore you should provide a script for locking/unlocking the container network (see below)
  • When restoring a container with veth device you may specify a name for the host-side veth device
  • In order to checkpoint and restore alive TCP connections you should use the --tcp-established option

Typically a container dump command will look like

crtools dump 
    --evasive-devices                # handle /dev/log usage bug
    --tcp-established                # allow for TCP connections dump
    -n net -n mnt -n ipc -n pid      # dump all the namespaces container uses
    --action-script "net-script.sh"  # use net-script.sh to lock/unlock networking
    -D dump/ -o dump.log             # set images dir to dump/ and put logs into dump.log file
    -t ${init-pid}                   # start dumping from task ${init-pid}. It should be container's init

and restore command like

crtools restore
   --evasive-devices
   --tcp-established
   -n net -n mnt -n ipc -n pid
   --action-script "net-script.sh"
   --veth-pair eth0=${veth-name}     # when restoring a veth link use ${veth-name} for host-side device end
   --root ${path}                    # path to container root. It should be a root of a (bind)mount
   -D data/ -o restore.log
   -t ${init-pid}

We also find it useful to use the --restore-detached option for restore to make contianer reparent to init rather than hanging on a crtools process launched from shell. Another useful option is the --pidfile one -- you will be able to find out the host-side pid of a container init after restore.

More details on the option mentioned can be found in Usage and Advanced usage pages.

Example

We have an application test for dumping/restoring an LXC Container. You may look at it for better understanding how to dump and restore your container with crtools.

This test contains two scripts:

run.sh
This is the main script, which executes crtools two times for dumping and restoring CT. It contains a working commands for dumping and restoring a container.
network-script.sh
This one is used to lock and unlock CT's network as described above.