CRIU - User contributions [en]

CLI/opt/--tcp-close

2017-07-21T14:15:10Z

Ebatalov:

''Main article: [[TCP connection]]''

tldr; This option is only used with CRIU restore command.
It allows your listening (server) TCP sockets to exist after process tree c/r. It drops all the other TCP connections in the process tree.
All your server clients will be able reconnect to its listening sockets created during process tree restore.
--tcp-close option allows you to migrate your servers to environments with different ip addresses if your sockets listen on addresses such as 0.0.0.0.
Moreover you'll be able to restore many copies of your server on different machines in this case.
--tcp-close option works well with another one that allows to change ip addresses of sockets in CRIU image files. It is not implemented yet (https://github.com/xemul/criu/issues/211).

Some details:

All the original process tree sockets data is preserved in process tree image files.
--tcp-close option affects all the process tree TCP sockets that have any TCP state except CLOSED and LISTEN.
Those affected sockets are restored in CLOSED TCP state (TCP_CLOSE Linux macro).
Userspace code that uses those affected sockets will just get errors from its syscalls. Then it would assume that there was a networking/peer problem
and it would execute usual error handling code path.

[[Category: CLI]]
[[Category:Sockets]]
[[Category:Editor_help_needed]]

CLI/opt/--tcp-close

2017-07-21T13:55:50Z

Ebatalov:

Final states

2016-08-26T18:35:39Z

Ebatalov:

= The concept of process tree final states =

Final state is the state that ps tree will have after its dump or restore.

There are 3 possible final states in CRIU:
# 'Running' (ps tree is executed as usual)
# 'Stopped' (ps tree is stopped using SIGSTOP)
# 'Dead' (ps tree is destroyed using SIGKILL)

= CLI options for changing of process tree final state =
You can use --leave-stopped option with CRIU dump/restore commands to set process tree final state to 'Stopped'.

Use --leave-running option to set final state to 'Running'.

= Process tree final state after CRIU dump command =

By default final state of process tree after dump is 'Dead'.
So CRIU will kill process tree after it was dumped. But why? Suppose you leave your
process tree running after it was dumped. What if it deleted some files it was using
during its dump? What if it closed some tcp connections? After that CRIU won't be able
to restore process tree from the dump it has created earlier.
System simply won't have resources that this process tree requires at
the moment captured in dump.

If your process tree doesn't destroy resources it depended on before dump then you'll be able to restore it after dump with --leave-running option.
But be aware that your process tree left running after dump may modify some state (for example a file) in way not compatible on application level with process tree restored from this dump later.

What about leaving process tree stopped after dump was performed? When it is needed?
It may be needed for debugging of CRIU. If CRIU wasn't accurate during dump command
then it would leave some traces in dumped process tree. You can investigate such process tree in
stopped state. You'll probably find some other use cases for --leave-stopped option with CRIU dump command.

Leaving process tree running is necessary for predump command and currently --leave-stopped and
--leave-running options have on effect on it.

= Process tree final state after CRIU restore command =
By default final state of process tree after restore is 'Running'. That's because you usually
want to immediately start execution of process tree after its restore. You can use --leave-stopped
option to restore process tree in 'Stopped' state.

= Moving process tree from 'Stopped' state to 'Running' =
After criu has dumped/restored process tree in 'Stopped' state we sometimes need to continue
its execution putting process tree into 'Running' state. For this purpose use this [https://github.com/xemul/criu-scripts/blob/master/pstree_cont.py script]. It has only single argument - PID of process tree root.

[[Category: Under the hood]]