Re: Modifying container storage for Fedora 26.

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On 11/16/2016 02:56 PM, Vivek Goyal wrote:
> On Wed, Nov 16, 2016 at 02:49:25PM -0500, Stephen Gallagher wrote:
>> On 11/16/2016 02:40 PM, Vivek Goyal wrote:
>>> On Wed, Nov 16, 2016 at 02:32:46PM -0500, Daniel J Walsh wrote:
>>>> We would like to change the docker container storage to default to
>>>> Overlayfs2 in Fedora 26.  But we have a problem on Atomic Host and
>>>> Fedora Server distributions.
>>>>
>>>>
>>>> Currently docker-storage-setup defaults to devicemapper and is hard
>>>> coded to setup a thinpool of 40% of remaining disk.  Otherwise it sets
>>>> up loopback devices on the root file system.   Devicemapper is nice
>>>> since it works with thinpools and can automatically expand the storage
>>>> if the disk space is getting used up. 
>>>>
>>>> Moving to Overlay, we can more easily use the root file system directly,
>>>> which would be fine for Fedora Workstation.  We want to preserve the use
>>>> of the remaining storage for Overlay on AH and Fedora Server,  since
>>>> this would give a user flexibility to switch back to using devicemapper
>>>> if they had problems with the Overlay driver.
>>>
>>> And being able to do so basically involves following.
>>>
>>> - docker-storage-setup creates a logical volume from free space
>>> - Creates a filesystem on that logical volume
>>> - Mounts that logical volume on the directory which docker is going to
>>>   use.
>>>
>>>   mount /dev/docker-vg/foo /var/lib/docker/
>>>
>>> - Now when docker users overlay2 graph driver, all the images, containers
>>>   and associated metadata will be stored outside the root filesystem and
>>>   onto /dev/docker-vg/foo logical volume.
>>>
>>>> We can not as easily
>>>> support the expanding disk for Overlay since we will not use using thinpool.
>>>
>>>>
>>>> We have looked at options to hard code OverlayFS with the defaults,
>>>
>>> If we always mount /var/lib/docker on on /dev/vg/foo for overlay2 driver
>>> this will be a regression w.r.t current behavior. So I would not
>>> recommend changing current behavior. I think this should be an opt-in.
>>> We are working on provide a config knob to elect this behavior and
>>> atomic host and fedora server will have to opt-in somehow.
>>>
>>> I think it will be easy for atomic host as they already drop something
>>> in /etc/sysconfig/docker-storage-setup. Not sure how fedora server
>>> variant will do it.
>>>
>>
>>
>> Today, Fedora Server relies on whatever is the default for docker-storage-setup.
>> We just tell Anaconda to reserve up to 15GiB by default for the / partition and
>> then it puts all remaining free space (on drives selected to be used by
>> Anaconda) into a single logical volume with no partitions.
>>
>> It's a very easy thing for us to drop a different config file for
>> docker-storage-setup into place for Server. So if that's all we need to do, let
>> me know and I'll work it up.
> 
> Ok, that sounds good. We are working on providing a knob to opt-in new
> behavior. I think all you have to drop in config file will be something
> like.
> 
> /etc/sysconfig/docker-storage-setup
> 
> STORAGE_DRIVER=overlay2
> YET_TO_BE_NAMED_OPTION=VAL
> 
> So upstream default will continue to be devicemapper. We will have to
> modify fedora workstation, fedora server and atomic host infrastructure
> to opt-in for overlay2.
> 

Why exactly does this need to be opt-in? Why wouldn't we just change the default
on Fedora Server to use overlay2 instead of devicemapper?

I think I'm missing some key part of the problem here.


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