We have a problem with btrfs subvolumes, as they can hold a filesystem which can be mounted, but aren't describable using only the name of a block device. Specifically they need at least a block device and a subvolume name. There are several APIs which operate on filesystems which currently describe it using only a block device. I've listed them all below. In all cases, these APIs need to use a more fully specified 'mountable' object. There are currently 2 cases we need to support: bare device, and device + subvolume. As this is a significant API change, I believe we should also allow the mountable description to be safely extended without breaking the api again. I propose a string which contains a descriptor followed by data custom to the descriptor. For the 2 existing cases this would be: device:/dev/sda3 subvol:/dev/sda1,root Any other solution must convey the same information. Could we achieve this with a union, for e.g.? How would this work with bindings? I would deprecate all of the apis below and replace them with alternative versions with a _ext suffix. The replacement apis would accept and return the enhanced descriptor. Inspection APIS: All take a root filesystem as an argument, which may not be a block device. inspect_get_arch inspect_get_distro inspect_get_drive_mappings inspect_get_filesystems inspect_get_format inspect_get_hostname inspect_get_icon inspect_get_major_version inspect_get_minor_version inspect_get_mountpoints inspect_get_package_format inspect_get_package_management inspect_get_product_name inspect_get_product_variant inspect_get_type inspect_get_windows_current_control_set inspect_get_windows_systemroot inspect_is_live inspect_is_multipart inspect_is_netinst inspect_list_applications inspect_list_applications2 The following return filesystem descriptors. inspect_get_filesystems inspect_get_roots inspect_os Mount APIS: All currently take a block device, but need to accept any filesystem descriptor. mount mount_options mount_ro mount_vfs Label APIs: All these currently take a block device, but are arguably applicable to btrfs subvolumes. A subvolume doesn't have a label itself, but its root filesystem does. A btrfs subvolume can still be mounted by label, as long as the subvolume is also given. set_label: note that this would also affect other subvolumes vfs_label vfs_type vfs_uuid Device name mangling: These apis are used to return sanitised device names. I'm not 100% convinced we need to modify them. However, we may want additional apis which handle filesystem descriptors. canonical_device_name part_to_dev Misc: zerofree: only actually supports ext2 and ext3, but the operation makes sense for any filesystem. We should update the api and consider the implementation later. findfs_label findfs_uuid: These return a device which contains the btrfs filesystem. This device will be mountable, even if the filesystem spans many devices. Not 100% convinced about this one, although it feels deficient.