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Device merging is used by EC2 API at least. This allows us (EC2 API team) to implement a use case of changing attached volume parameters when launching an instance from a volume backed image. If the image's bdms contain several volume descriptions, but a user wants to change some of them for a new instance, he can [1] E.g. the user may want to increase root volume size, skip another volume.


Nova API doesn't support this ability fine (we must specify full bdm rather than changes only), but it works at least. If you remove device names from API, what alternative way are you going to provide instead for this purpose?


[1] http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html#Using_OverridingAMIBDM


Thanks,

Feodor Tersin.

________________________________
From: Matt Riedemann <mriedemos at gmail.com>
Sent: Sunday, April 2, 2017 6:59:12 PM
To: Matt Riedemann; openstack-dev at lists.openstack.org; openstack-operators at lists.openstack.org
Subject: Re: [openstack-dev] [nova] Removing BDM devices from POST requests

On 4/2/2017 10:13 AM, Matt Riedemann wrote:
> On 4/1/2017 1:01 PM, Matt Riedemann wrote:
>> I know we've talked about this over and over and another bug [1]
>> reminded me of it. We have long talked about removing the ability to
>> specify a block device name when creating a server or attaching a volume
>> because we can't honor the requested device name anyway and trying to do
>> so just causes issues. That's part of the reason why the libvirt driver
>> stopped honoring the block device name in requests back in Liberty [2].
>>
>> I think we all agree on removing the device name from the API, but I'm
>> having a hard time remembering if someone signed up to write a spec for
>> this. I could have sworn this came up recently and someone said they'd
>> write a spec, but I can't remember.
>>
>> So this is my attempt at remembering and if it's all a dream, then is
>> anyone interested in owning this? If not, I'll start writing the spec
>> this week.
>>
>> [1] https://bugs.launchpad.net/nova/+bug/1648323
>>
>
> One thing I thought about with this is if you want to identify the block
> device after it's attached to the server, you can use device tags to do
> that (introduced in microversion 2.32). But that's only at server create
> time until [1] is merged. So removing the device from attach volume
> requests is probably dependent on completing Artom's blueprint first for
> tagging volumes when you attach them to an existing server.
>
> [1]
> https://specs.openstack.org/openstack/nova-specs/specs/pike/approved/virt-device-tagged-attach-detach.html
>
>

Spec started here:

https://review.openstack.org/#/c/452546/

--

Thanks,

Matt

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