What cannot be checkpointed

CRIU cannot dump all the possible states applications can live in. This article summarizes our experience in that area.

External resources

By default CRIU allows to dump the set of processes and their resources if this set has no connections outside. However, in some situations it makes sense to ignore this external connection on dump and recreate one on restore. So, what this "connection outside" is? For example:

UNIX socket

Application may have a UNIX socket connected to some other app and the latter one is not dumped. This is called external UNIX socket.

TTY, group and session

If you start a program from shell, the tty, process group and session of the new program can be shared with the shell itself. People often meet this when they try CRIU for the first tome on a simple loop.

TCP connection

This socket is literally an external connection, so CRIU should be explicitly allowed to dump one.

File locks

A file lock is an object, that belongs to some filesystem. On dump it's impossible to find out whether this lock help by one task can be used by some other. Thus, CRIU doesn't dump tasks with held locks. The --file-locks CLI option tells CRIU to dump the lock.

Devices

If an task has opened or mapped any character or block device, this typically means, it wants some connection to the hardware. In this case dump (and restore) is impossible. The exception is virtual devices like null, zero, etc. and TUN network device (used by OpenVPN).

Tasks with debugger attached

CRIU uses the same API as debuggers do to get some tasks' state and this API (the ptrace one) doesn't allow for multiple debuggers to explore a task. Thus tasks under gdb or strace cannot be dumped.