providers/aws: add root_block_device to aws_instance #998
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AWS provides a single
BlockDeviceMapping
to manage three differentkinds 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
, anddelete_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 specifyvolume_size
,volume_type
,delete_on_termination
,snapshot_id
, andencryption
.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 Ihave yet to see ephemeral block devices working properly.
Looking into possibly separating out
ephemeral_block_device
andebs_block_device
sub-resources as well, which seem like the logicalnext 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