maas 2.7 regression: creates partition when trying to use entire disk as bcache cache device

Here is the same thing on 2.6:

before creating:
http://paste.ubuntu.com/p/7bCDdwwb4p/

creating and result:
2020-03-17-18:52:21 root DEBUG [localhost]: maas root bcache-cache-sets create r448pr cache_device=24
> /home/ubuntu/cpe/foundation/foundationcloudengine/foundationcloudengine/configurenodes.py(447)setup_bcaches()
-> backing_device = bcache["backing_device"]
(Pdb) cache_set
{'cache_device': {'firmware_version': None, 'serial': 'CVMD552400AX400AGN', 'size': 400088457216, 'tags': ['ssd'], 'model': 'INTEL SSDPEDME400G4', 'available_size': 0, 'used_size': 400088457216, 'uuid': None, 'partition_table_type': None, 'id_path': '/dev/disk/by-id/nvme-nvme.8086-43564d44353532343030415834303041474e-494e54454c205353445045444d453430304734-00000001', 'storage_pool': None, 'path': '/dev/disk/by-dname/nvme0n1', 'partitions': [], 'type': 'physical', 'used_for': 'Cache device for cache0', 'block_size': 4096, 'system_id': 'r448pr', 'filesystem': {'fstype': 'bcache-cache', 'label': None, 'uuid': '344c0027-069d-4473-9957-93437754cfa3', 'mount_point': None, 'mount_options': None}, 'name': 'nvme0n1', 'id': 24, 'resource_uri': '/MAAS/api/2.0/nodes/r448pr/blockdevices/24/'}, 'system_id': 'r448pr', 'name': 'cache0', 'id': 1, 'resource_uri': '/MAAS/api/2.0/nodes/r448pr/bcache-cache-set/1/'}

block-devices read shows the cache device is the entire disk and there are no partitions/partition table.
http://paste.ubuntu.com/p/ZH8h2sWTdg/

When you create a bcache, MAAS will always create a partition on the underlying block device IF it's the "boot device" - see bug 1509536.

When MAAS doesn't know the boot device, it falls back on the first physical block device created and assumes that is the boot device.

1) can you confirm what MAAS has listed as the physical block devices on the node?
2) Are you able to tell MAAS what the boot disk is? e.g. `maas root block-device set-boot-disk r448pr <block-device-id-of-boot-disk>`

1) We aren't using bios, so "boot device" shouldn't matter. We're explicitly telling MAAS where to put EFI and it isn't here.
2) This is a behavior change from 2.6 - a regression.

I have spent a bunch of time trying to resolve this bug and I've gotten a little stuck. I wanted to write up a bit about what I have learned so far.

There is a method in maasserver.models.node, get_boot_disk which returns the boot disk for a node. However, if there isn't one specified then it will default to the first physical disk. This seems like a confusing behaviour and I think it is the cause of the issue here. There is also another suspicious method is_boot_disk in maasserver.models.blockdevice, which uses get_boot_disk. This means that if there is no specified boot disk, but if there is a physical disk that can be found then is_boot_disk will return True.

I was able to replicate this bug with this unit test:

+ def test_create_without_creating_partitions(self):
+ """Verify we don't create a partition when creating a bcache cache set
+
+ Test for LP 1867812
+ """
+ self.become_admin()
+ node = factory.make_Node(
+ status=NODE_STATUS.READY, with_boot_disk=False
+ )
+ uri = get_bcache_cache_sets_uri(node)
+ cache_device = factory.make_PhysicalBlockDevice(node=node)
+ response = self.client.post(uri, {"cache_device": cache_device.id})
+ self.assertEqual(
+ http.client.OK, response.status_code, response.content
+ )
+ parsed_device = json_load_bytes(response.content)
+ self.assertFalse(
+ parsed_device["cache_device"]["partitions"],
+ )

This test causes get_boot_disk to be called from three different places.

  File "/home/ubuntu/code/canonical/maas/src/maasserver/models/blockdevice.py", line 302, in create_partition_if_boot_disk

  File "/home/ubuntu/code/canonical/maas/src/maasserver/models/partition.py", line 223, in get_partition_number

  File "/home/ubuntu/code/canonical/maas/src/maasserver/storage_layouts.py", line 59, in __init__

When trying to remove these three uses I broke a test in maasserver.forms.tests.test_cachset; TestCreateCacheSetForm.test_cache_set_creation_with_boot_disk. This test seems to verify the opposite behaviour and expects partitions to be created. Despite the node not having a explicit boot disk assigned.

Looking at when this test had that behaviour introduced, it was in this 2015 change: https://git.launchpad.net/maas/commit/?id=035e8d44c07aef638dae9579627e67129aa3cb27

This makes me wonder how the 2.6 behaviour was ever possible. It might be that I am missing something or my reproduction is incomplete.

From a brief discussion on mattermost it sounds like the best solution here would be to make everything much more explicit. By having users specify which disks should be bootable and we should be clear when partitions will be created.

I have also spent some time diving into the git history to try and track down whatever the breaking change might be. No luck there. I'd really like to better understand why/when the behaviour changed.

sub'd to field-high; field team is hitting this too.

Does this issue still happen on MAAS 3.2 or later?

Closing as outdated and no longer reproducible