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
Some instance types have a block device by default. So when selecting
such an instance type, you will not set a config for the block device,
but the update/refresh func will notice one and update the state
nonetheless.
So in those cases the `block_device` becomes a `computed` field.
1. The schema contained a few fields that where not marked as
`computed`, while they were updated inside the resource.
2. While updating the `volume_size` it was doing so with a `string`,
but in the schema this field is set as `int`.
3. The set func for calculating the hashes for the `block` set items,
also used computed values to calculate the hash. As these values will
not be in the config, but only in the state, this will always show as a
diff. The solution is to only use the fields that aren’t computed in
order to get consistent hashes.
These where all issues before, but weren’t visible as such. All should
be good again now.
Mitchell Hashimoto