RFC: unmounting /mnt on AWS instances

[Andrew Wilkins]

On Tue, Nov 18, 2014 at 10:58 PM, David Britton <david.britton at canonical.com
> wrote:

>
> On Mon, Nov 17, 2014 at 8:25 PM, Marco Ceppi <marco at ondina.co> wrote:
>
>> I have a few charms that expect /mnt to be the ephemeral disk, while it
>> wouldn't be a huge headache and I certainly
>
>
> I'm in the same boat, it won't be a huge headache, but it will be work to
> do.  I've always thought that putting the ephemeral on /mnt was a bit of an
> odd decision, but it's not a killer.  Some things I've thought of:
>
> If you leave things the way they are, you can still use /mnt as it is.
> The ephemeral survives reboots, so any persistent dirs put in there would
> be fine and work seamlessly to the user.  Also if people wanted to continue
> to write out larger files to /mnt, that would work fine as well.
>
> If you remove the behavior, it would be good to have it on
> /mnt/juju-ephemeral-0  or something like that.
>

The way things are intended to work (subject to change), is that Juju will
periodically list disks and whether or not they are in use (mounted). If
they're not in use, then the Juju user can assign the disk to a unit that
wants storage. So it's not a problem that it's in /mnt, but a matter of
whether we can not mount it at all. We'll also enable requesting/assigning
disks at instance provisioning time, and dynamically post-provisioning.

In terms of changing charms, I think at worst you'd have to add something
like this to your charm metadata:
"""
storage:
    arbitraryname:
        type: filesystem
        location: /mnt
        required: true
"""

Then when you deploy to AWS, you should be able to do this:
    $ juju deploy mycharm --storage arbitraryname=ephemeral0
or
    $ juju deploy mycharm --storage arbitraryname=100G,iops:10000
or ...

Ideally that would not be "/mnt", because otherwise we're going to end up
with a lot of charms that cannot be co-located. My point is that it can be
/mnt, and your charm will keep working without any changes to hooks.

Cheers,
Andrew
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