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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 |
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