Hello Neil,

first, many many thanks for all your good work on MD and always helping
us here on this list!

On Tue, 23 Jul 2013, NeilBrown wrote:

As you probably know, when you use "mdadm -C" to create an array, it will
check if the devices appear to contain a filesystem or similar already and
will complain if they do - requiring you do say "yes" or use "--run" to avoid
the warning.

However if you use "--add" to add a device to an existing array no such
checks are done.  So it isn't too hard to destroy all those cat photos you
have saved on a USB drive (because device names change every time you boot
and you got confused).


I could easily change "--add" to be more cautious, but that might break
existing scripts, which I would rather not do.

Or I could add a "policy" line to mdadm.conf which would indicate the policy
for "--add" - either "spare" or "force-spare".  But then I would need to
decide on a default.  The default should probably be safe otherwise people
probably won't change it until they get burned.  So people with scripts would
still experience breakage, but could now fix it easily with a "policy" line.

Or maybe "--add" should be deprecated so people have to choose between
"--re-add" or a new "--spare" with --spare requiring "--force" to destroy
data.
Then "--add" would generate a deprecation message which scripts could  ignore
but people might learn from.


I don't think there is an obviously-correct answer here so I'm open to
suggestions.  What do people think?

I first thought of a --initialize, but I do not think it is any better.
So I would vote for the second solution, depreciating --add and using
--spare plus --force.

Holger