Live migration

The crtools utility can be used to perform live migration of apps or containers. This page is a sort of HOWTO describing this.

In order to live-migrate an application or a container you should make sure, that files, that are/can be accessed by processes you're migrating are available on both nodes -- source and destination. This can be achieved by using either shared file-system such as NFS, GlusterFS or CEPH, or by using rsync to copy files from one box to another. Further in this article we assume, that the file-system is the same on both sides.

So, in order to live migrate tasks you should do these steps:

step 1

Take tasks you're about to migrate and dump them into some place, asking crtools to leave them in stopped state after dump:

# crtools dump --tree <pid> --images-dir <path-to-existing-directory> --leave-stopped

The directory you put images to can reside on the shared file-system if you're using one. In this case you can proceed to step 3.

step 2

Copy images to destination node

# scp -r <path-to-images-dir> <dst-node>/<path-to-images>

step 3

Go to the destination node and restore the apps from images on it

# crtools restore --tree <pid> --images-dir <path-to-images>

step 4

If everything went OK you can return on the source node and kill stopped tasks on it.

The directories with images would contain two copies of applications memory, which may be space-consuming. The CRIU can perform disk-less migration to address this.