Comment 2 for bug 1960231

The apparmor profile would definitely need to be fixed if it's blocking the mkfs operations. However, in terms of the default format, there seems to be a following logic[1].

If mkfs_command nor explicit fs is specified, then default_ephemeral_format (None in the upstream, ext4 in the charm[2]). If default_ephemeral_format doesn't have a value, then the code looks for os_type and set the default fs as windows=ntfs, linux=ext4, and if there is no os_type specified, it falls back to vfat.

default_ephemeral_format=ext4 in the charm default might be too opinionated since the following fallback logic per OS in the upstream is always skipped.

_DEFAULT_FS_BY_OSTYPE = {'linux': FS_FORMAT_EXT4,
                         'windows': FS_FORMAT_NTFS}

[1] https://github.com/openstack/nova/blob/stable/ussuri/nova/privsep/fs.py#L290-L305
[2] https://bugs.launchpad.net/charm-nova-compute/+bug/1693943