Iterative migration

This page describes how to reduce the freeze time of an application by using the memory changes tracking ability.

Note: It is assumed that you already read Live migration article before this one.

Migration sequence

The steps below look like those in regular live migration, but include one of more pre-dump stages.

Pre-dump

Take tasks you're about to migrate and pre-dump them into some place. Tasks will remain running after pre-dump, unlike regular dump.

[src]# criu pre-dump --tree <pid> --images-dir <path-to-existing-directory-A>

The directory with images can be on a shared storage, or you can use disk-less migration to avoid the Copy step.

Now you can either proceed to next step and do regular dump, or perform the pre-dump step again. In the latter case pre-dump would generate another set of pre-dump images which will contain memory, that has changed from previous step. Doing several pre-dump stages (or iterations) may reduce the amount of data dumped on dump stage and thus -- smaller freeze time.

Dump

Now you can do regular dump of your processes.

[src]# criu dump --tree <pid> --images-dir <path-to-existing-directory-B> --prev-images-dir <path-to-directory-A-relative-to-B> --leave-stopped

Note, that

this dump would work faster, than it would if there were no the pre-dump stage, as this dump only takes the memory that has changed, since the pre-dump step.
the --prev-images-dir should contain path to the directory with pre-dump images relative to the directory where the dump images will be put

Copy

Copy images to destination node:

[src]# scp -r <path-to-images-dir> <dst>:/<path-to-images>

Restore

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

[dst]# criu restore --tree <pid> --images-dir <path-to-images>

Kill

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

[src]# FIXME put command here