Re: question: no-bitmap RAID1 with off-site drive

At 01:31 PM 11/27/2012 +1100, NeilBrown wrote:

quoted

Appears then that 'mdadm' may not
check the superblock to see if the
drive came from a different array
and should not be overwritten, and
instead prefers to just zap it.

Correct.  When you ask mdadm to --add device to
an array, that is what it will do.  If the
metadata looks particularly good it might do a 
re-add for you, but if not it will just erase
it and write some more.

Thank you very much for the confirmation.

quoted

Can anyone confirm or deny the
possibility?  I can manually run
an --examine and check the array
name as a precaution when rotating
drives if 'mdadm' doesn't perform
the check.  Want to avoid inserting
a offsite drive in the wrong array.

We might be able to make "mdadm -I" do what
you want ... find out which array
it "should" be a member of and auto-add it.
But that will currently fail for an
out-of-date member with no bitmap.

Actually I was thinking more about 'mdadm'
not destroying an old-but-good mirror
copy from a different array.  So if I
accidently pull the wrong drive out of
the safe deposit box, then 'mdadm' would
refuse to zap and re-label the drive on
an attempt to insert it in the wrong
array.  An explicit --zero-superblock
would have to be run before a drive could
be re-used in any array with a different
name.

Not a big deal though.  I will be sure
to --examine all rotate-in drives
carefully before --grow --add'ing them.