Re: Should "mdadm --add" complain if the new device appears to have a filesystem on it?

[Christian Iversen <chrivers@iversen-net.dk>]

On 2013-07-23 09:41, Holger Kiehl wrote:
> 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.

If we have to change the options anyway, then I'm more in the mood for 
--add --force for new volumes, and --add for existing ones. It would be 
more in line with what we have now, IMHO.

(please CC, not on the list)

-- 
Med venlig hilsen
Christian Iversen