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The following instructions describe the process to build/install components necessary to checkpoint/restart (C/R) LXC containers using the C/R implementation being pushed into mainline.

The three main components to be built/installed are:

  • C/R enabled Linux kernel
  • USERCR - the user-space component of checkpoint/restart
  • LXC

Each of these components must currently be built from source from their git trees. The commit ids for each of these must be in sync for the entire "stack" to work correctly. In time, as new fixes go into each of these trees, new "stacks" become available. Choose a stack based on the LXC release below and use the commit ids from that stack in the build instructions to build the stack.

LXC 0.6.5 based stack

   - LXC_RELEASE = 0.6.5
   - KERNEL_COMMIT_ID = 0fdca57255b8b5bc9a4f107bee7f1e47d2630cb3
   - USERCR_COMMIT_ID = fa382a1758ef395858b1bea530ccdb4e1360b76f
   - LXC_COMMIT_ID = f78a1f32f41f6acbbf0b78e6498736dbd22e2301

Verify commit ids for LXC_RELEASE 0.6.5

You can use the following additional description to verify the above commit ids for LXC_RELEASE 0.6.5.

   $ (cd linux-cr && git log --pretty=short -1 $KERNEL_COMMIT_ID)
   commit 0fdca57255b8b5bc9a4f107bee7f1e47d2630cb3
   Author: Dan Smith <>
       Disable softirqs when taking the socket queue lock
   $ (cd user-cr && git log --pretty=short -1 $USERCR_COMMIT_ID)
   commit fa382a1758ef395858b1bea530ccdb4e1360b76f
   Author: Sukadev Bhattiprolu <>
       Add keep_frozen field to struct app_restart_args
   $ (cd lxc-git && git log --pretty=short -1 $LXC_COMMIT_ID)
   commit f78a1f32f41f6acbbf0b78e6498736dbd22e2301
   Author: Daniel Lezcano <>
       fix when console is not specified

LXC-0.7.1 based stack

   - LXC_RELEASE = 0.7.1

Note: This page is still under construction. Feel free to fix obvious mistakes or email

Basic debugging of C/R

There is a strong dependency between each of these components (USERCR and Linux kernel are tightly coupled as are USERCR and LXC) so it is important to ensure the versions match up correctly. Otherwise attempts to checkpoint/restart typically fail with terse -EINVAL or -EBUSY errors.

If this happens, usually more debug information is found in dmesg output. If lxc-checkpoint fails some additional error information can be found by running

  • $ /bin/ckptinfo -ev <checkpoint-statefile>


Statefile / checkpoint-image - this refers to the file in which the application state is saved after a successful checkpoint.

LXC - This refers to implementation of linux containers. Specifically this refers to the git tree git://

USERCR - The user-space commands/library components that utilize the kernel system calls to checkpoint/restart applications. Specifically this refers to the git tree: git://

1. Build C/R-enabled Linux kernel

Get the C/R-enabled kernel sources

   $ cd /root
   $ git clone  git:// linux-cr
   $ cd linux-cr
   $ git checkout -b test-1 $KERNEL_COMMIT_ID

Ensure following tokens are set in .config


To work around build issues with unexported symbols reported against this kernel version, ensure that the following symbols are each y or n, but not m: CONFIG_IPV6, CONFIG_MACVLAN, and CONFIG_VETH.

Build, install, reboot on new kernel

After every reboot, ensure '-o newinstance' mount option to /dev/pts works (see Documentation/filesystems/devpts.txt for details). In short, run following commands on each reboot:

  $ rm /dev/ptmx
  $ ln -s pts/ptmx /dev/ptmx
  $ chmod 666 /dev/pts/ptmx

Disable nscd if it is running -- nscd can cause file descriptors to be passed between mount namespaces, which is not supported by the current C/R code.

2. Build/install USERCR

Get latest USERCR sources

   cd /root
   $ git clone git:// user-cr
   $ cd user-cr
   $ git checkout -b test-1 $USERCR_COMMIT_ID

Apply any USERCR patches

If any patches are needed for the USERCR tree, they will be available in $LXC_RELEASE/usercr-patches directory of:, where LXC_RELEASE is set above.

Get those patches, and use, say, git am -3 to apply each of the patches in the directory to the USERCR tree.

Build and install USERCR binaries

Build USERCR by pointing to corresponding kernel-source tree we used above and install binaries. This should create restart.o and checkpoint.o needed by LXC, which, for now, are left in the current directory.

Note: You may need to compile checkpoint.o and restart.o with -fPIC compiler option.

   $ KERNELSRC=/root/linux-cr make 
   $ ls restart.o checkpoint.o
   restart.o   checkpoint.o
   $ make install

3. Build/install LXC

Get latest LXC sources

   $ cd /root
   $ git clone git:// lxc.git
   $ cd lxc.git
   $ git checkout -b test-1 $LXC_COMMIT_ID

Apply LXC patches

Get the patch set from $LXC_RELEASE/lxc-patches directory of, where LXC_RELEASE is set above

Apply each of the patches in the directory to the LXC git tree

   $ git am -3 lxc-patches/0001-patch
   $ git am -3 lxc-patches/0002-patch

Configure, build, install LXC binaries

Configure/build/install LXC binaries using following commands. The configure command will fail if /root/user-cr specified below, does not contain checkpoint.o, restart.o and app-checkpoint.h files

   $ ./
   $ ./configure --with-libcr=/root/user-cr
   $ make
   $ make install

4. Checkpoint/restart a simple LXC container

   $ lxc-execute --name foo --rcfile lxc-no-netns.conf -- /bin/sleep 1000
   $ lxc-checkpoint --name foo --statefile /root/lxc-foo.ckpt
   $ lxc-stop --name foo
   $ lxc-restart --name foo --statefile /root/lxc-foo.ckpt
   $ lxc-stop --name foo

5. Checkpoint/restart an LXC container running a VNC server

VI editing session in a VNC session

Run a "vi" editing session inside a VNC server using "twm" window manager

   $ cat /root/.vnc/xstartup
   xsetroot -solid grey
   vncconfig -iconic &
   xterm -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
   twm &
   $ lxc-execute --name foo --rcfile lxc-no-netns.conf -- /usr/bin/vncserver :1
   $ vncviewer :1
   # Open a vi session in vnc viewer
   $ lxc-checkpoint --name foo  --statefile /root/vnc.ckpt
   $ lxc-stop --name foo
   $ lxc-restart --pause --name foo --statefile /root/vnc.ckpt
   # Leaves the server frozen due to --pause
   $ lxc-unfreeze --name foo
   $ vncviewer :1
   # Should bring up the old VNC session with vi window
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