[Devel] Any plans about overlayfs support for Docker?

[Konstantin Khorenko]

On 05/24/2015 02:24 PM, Pavel Odintsov wrote:
> Hello, folks!
> 
> I inspired with Docker support inside containers so much! It works perfectly.
> 
> But I can't find any articles regarding to OverlayFS support there.
> 
> Do you have any plans about OverlayFS
> http://blog.cloud66.com/docker-with-overlayfs-first-impression/?
> 
> It's must feature for Docker and it's pretty useless without it :(

Hi Pavel,

yes, currently Docker running inside an OpenVZ (and Virtuozzo) Container can use only "vfs" graph driver.
Yes, we know that "vfs" driver consumes a lot of diskspace and coping all the data on Docker CT creation
makes this operation quite slow.

So what are we going to do with that?
In fact nowadays the only alternative which is stable enough is "devicemapper",
but we cannot just let Docker to use it inside an OpenVZ CT because if we do and allow OpenVZ owner to write to
the image and later mount it - the owner can easily write garbage and crash the kernel on a mount attempt.

To avoid this we are working on another simple graph driver - "proxy".
Basically it will redirect all requests from inside the OpenVZ Container to a daemon running on Host which will prepare disk for Docker Container from outside.

Once we have a prototype of it we'll push it to Docker mainstream.

Returning to the question about OverlayFS:
* it's not that simple to be bug free in the near future
* it's appeared quite recently => porting to 2.6.32-x may be a pain

=> we don't plan to add OverlayFS support in RHEL6-based (2.6.32-x) OpenVZ kernels.


On the other hand it's quite possible that OverlayFS will become good and stable enough and we'll eventually add it to Virtuozzo 7 (RHEL7-based) kernel one day.

Hope that helps.

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team