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.|
The steps below look like those in regular live migration, but include one or more pre-dump stages.
Take tasks you are 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 changed after previous pre-dump. Doing several pre-dump iterations may reduce the amount of data dumped on dump stage and thus lead to shorter freeze time.
Note, that if you're going to perform more than one pre-dump steps, you should create different directories for images and properly reference them with the
--images-dir and the
--prev-images-dir for all pre-dump and dump steps.
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
- this dump would work faster than without pre-dump, as this dump only takes the memory that has changed since the last pre-dump;
--prev-images-dirshould contain path to the directory with pre-dump images relative to the directory where the dump images will be put.
Copy images to the destination node:
[src]# scp -r <path-to-images-dir> <dst>:/<path-to-images>
On the destination node restore the apps from images:
[dst]# criu restore --tree <pid> --images-dir <path-to-images>
If everything went OK you can kill stopped tasks on the source node:
[src]# FIXME put command here