[systemd-devel] systemd and criu (checkpoint / restart)
alex at polvi.net
Tue Jul 2 17:42:00 PDT 2013
TL;DR criu works if you disable the journal and stop the .socket
before restore, criu appears to be incompatible with systemd-nspawn.
I've been "having fun" with systemd, -nspawn, and the latest criu
tools. These are just my research notes. I wanted to share progress,
would love any feedback or pointers on places this does not work.
Before we begin:
localhost criu2 # systemd --version
+PAM -LIBWRAP -AUDIT -SELINUX +IMA -SYSVINIT -LIBCRYPTSETUP -GCRYPT -ACL -XZ
localhost criu2 # criu -V
localhost criu2 # uname -a
Linux localhost 3.10.0+ #4 SMP Mon Jul 1 13:36:25 PDT 2013 x86_64
Intel(R) Core(TM) i7-2677M CPU @ 1.80GHz GenuineIntel GNU/Linux
In the following gist:
1) I setup a socket activated go http server (just plain, no nspawn)
2) start the process via socket activation
3) criu dump it
4) shut down the .socket
5) criu restore, works, yay
- Application state is dumped and restored successfully.
What doesn't work:
- The system .socket has to be disabled to because the restore will
open the socket. Not sure if there is a work around for this, with the
exception of not using socket activation.
- The status of the .service is now in a killed state. This is because
the dump kills the process when it is done.
- Once the process is restored (with the same pid) systemd is confused
and is no longer monitoring it. Maybe there is a way to get systemd to
realize that the process is running again?
- I had to set StandardError=null and StanardOut=null to keep the
journal from opening a socket to the service. With-out this the
container will not checkpoint, because the criu tools do not allow one
end of a socket to be checkpointed.
For -nspawn, I'm pretty sure it is incompatible with criu. This took a
bunch of fighting, and in the end, triggered a bug in criu. I was able
to dump, but not restore the container.
In the following gist you'll see where I hit bugs and the fix...
1) I start a busybox while loop in a container using systemd-nspawn
2) nsenter the container and umount /proc/sys/kernel/random/boot_id
and /proc/kmsg because they are in a "(deleted)" state, and criu does
not really care for that. I guess -nspawn is setting these up.
3) Updated my copy of iproute2, because criu requires the "ip addr
4) Now it gets really bad... criu restores mount namespaces using
pivot_root. systemd uses MS_ENTER. Initially I was hitting a bug
where pivot_root was not working because my container was on a
different filesystem. After bind mounting the container filesystem to
my running root, the restore triggers a bug.
5) I give up.
- It dumped the state of something!
What does not work:
- Using the containers mount namespace (because of criu and pivot_root)
In summary, to make this actually work, I think we'd need to implement
checkpoint/restart into systemd itself. With this, we could get around
all the journal issue, and maybe even make socket activation work.
Containers seem to be their own beast.
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