Comment # 9 on bug 1205740 from David Sterba

This is not a bug. You seem to be preoccupied with concepts, terms and
expectations from another filesystem, my guess is ZFS. You don't need to be a
developer to distinguish a mount point, subvolume or a filesystem. If you start
a task on a read-only mount it will fail, because that's the semantics of it. I
would find confusing if it would work - read-only mount but I can do a write
operation?

There's documentation explaining the basic concepts, if you find that it's not
sufficient in the level of detail please suggest what you read and expect
there, I'll update it. The concepts of subvolume in btrfs, LVM, zfs are
different and people have been indeed confused at first, it depends which was
the first encounter.

If you're looking for a single administrative point, then there's no such
thing. There are perhaps access points to the filesystem that are represented
by mount points with respective RW/RO access, and the / is the most common one
(but it's not necessarily the toplevel subvolume).

Transactional server is a particular use case that is not a typical one, eg.
because of a read-only mount. If you're doing manual changes to the layout you
could expect some resistance, eg. to avoid accidental errors or getting out of
sync with the system configuration.

You could go complain to TU administrative tools that they maybe don't allow to
change the raid levels. There it would be one more RW mount, rebalance, done.
The idea of the 'btrfs' tool is to be a building block for applications or use
cases. Once there's another layer that manages the system then it's better to
work from there, or with enough understanding and risk do it manually.