I was working on building a validation to check the user-provided
"device_name" for "root_block_device" on AWS Instances, when I realized
that if I can check it, I might as well just derive it automatically!
So that's what we do here - when you customize the details of the root
block device, device name is just comes from the selected AMI.
Instance block devices are now managed by three distinct sub-resources:
* `root_block_device` - introduced previously
* `ebs_block_device` - all additional ebs-backed volumes
* `ephemeral_block_device` - instance store / ephemeral devices
The AWS API support around BlockDeviceMapping is pretty confusing. It's
a single collection type that supports these three members each of which
has different fields and different behavior.
My biggest hiccup came from the fact that Instance Store volumes do not
show up in any response BlockDeviceMapping for any EC2 `Describe*` API
calls. They're only available from the instance meta-data service as
queried from inside the node.
This removes `block_device` altogether for a clean break from old
configs. New configs will need to sort their `block_device`
declarations into the three new types. The field has been marked
`Removed` to indicate this to users.
With the new block device format being introduced, we need to ensure
Terraform is able to properly read statefiles written in the old format.
So we use the new `helper/schema` facility of "state migrations" to
transform statefiles in the old format to something that the current
version of the schema can use.
Fixes#858
AWS provides a single `BlockDeviceMapping` to manage three different
kinds of block devices:
(a) The root volume
(b) Ephemeral storage
(c) Additional EBS volumes
Each of these types has slightly different semantics [1].
(a) The root volume is defined by the AMI; it can only be customized
with `volume_size`, `volume_type`, and `delete_on_termination`.
(b) Ephemeral storage is made available based on instance type [2]. It's
attached automatically if _no_ block device mappings are specified, and
must otherwise be defined with block device mapping entries that contain
only DeviceName set to a device like "/dev/sdX" and VirtualName set to
"ephemeralN".
(c) Additional EBS volumes are controlled by mappings that omit
`virtual_name` and can specify `volume_size`, `volume_type`,
`delete_on_termination`, `snapshot_id`, and `encryption`.
After deciding to ignore root block devices to fix#859, we had users
with configurations that were attempting to manage the root block device chime
in on #913.
Terraform does not have the primitives to be able to properly handle a
single collection of resources that is partially managed and partially
computed, so our strategy here is to break out logical sub-resources for
Terraform and hide the BlockDeviceMapping inside the provider
implementation.
Now (a) is supported by the `root_block_device` sub-resource, and (b)
and (c) are still both merged together under `block_device`, though I
have yet to see ephemeral block devices working properly.
Looking into possibly separating out `ephemeral_block_device` and
`ebs_block_device` sub-resources as well, which seem like the logical
next step. We'll wait until the next big release for this, though, since
it will break backcompat.
[1] http://bit.ly/ec2bdmap
[2] http://bit.ly/instancestorebytypeFixes#913
Refs #858
The Terraform configuration syntax defines what arrays are.
Use the word array consistently throughout the documentation
instead of list.
The corresponding JSON datatype is called array as well, and
since the Terraform configuration syntax is interoperable with
JSON it makes sense to use the term array to describe them.
Paul Hinze