How persistent are docker data-only containers

问题:

I'm a bit confused about data-only docker containers. I read it's a bad practice to mount directories directly to the source-os: https://groups.google.com/forum/#!msg/docker-user/EUndR1W5EBo/4hmJau8WyjAJ

And I get how I make data-only containers: http://container42.com/2014/11/18/data-only-container-madness/

And I see somewhat similar question like mine: How to deal with persistent storage (e.g. databases) in docker

But what if I have a lamp-server setup.. and I have everything nice setup with data-containers, not linking them 'directly' to my source-os and make a backup once a while..

Than someone comes by, and restarts my server.. How do I setup my docker (data-only)-containers again, so I don't lose any data?

回答1:

Docker containers will persist on disk until they are explicitly deleted with docker rm. If your server restarts you may need to restart your service containers, but your data containers will continue to exist and their volumes will be available to other containers.

回答2:

Actually, even though it was shykes who said it was considered a "hack" in that link you provide, note the date. Several eons worth of Docker years have passed since that post about volumes, and it's no longer considered bad practice to mount volumes on the host. In fact, here is a link to the very same shykes saying that he has "definitely used them at large scale in production for several years with no issues". Mount a host OS directory as a docker volume and don't worry about it. This means that your data persists across docker restarts/deployments/whatever. It's right there on the disk of the host, and doesn't go anywhere when your container goes away.

I've been using docker volumes that mount host OS directories for data storage (database persistent storage, configuration data, et cetera) for as long as I've been using Docker, and it's worked perfectly. Furthermore, it appears shykes no longer considers this to be bad practice.

回答3:

docker rm alone doesn't remove the actual data (which lives on in /var/lib/docker/vfs/dir)

Only docker rm -v would clear out the data as well.

The only issue is that, after a docker rm, a new docker run would re-create an empty volume in /var/lib/docker/vfs/dir.
In theory, you could with symlink redirect the new volume folders to the old ones, but that supposes you notes which volumes were associated to which data container... before the docker rm.

回答4:

It's worth noting that the volumes you create with "data-only containers" are essentially still directories on your host OS, just in a different location (/var/lib/docker/...). One benefit is that you get to label your volumes with friendly identifiers and thus you don't have to hardcode your directory paths.

The downside is that administrative work like backing up specific data volumes is a bit of a hassle now since you have to manually inspect metadata to find the directory location. Also, if you accidentally wipe your docker installation or all of your docker containers, you'll lose your data volumes.