Commits on Feb 18, 2015

1. providers/aws: add root_block_device to aws_instance …

   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/instancestorebytype
   
   Fixes #913
   Refs #858

   

   phinze committed Feb 18, 2015
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