It is time to expand the family's backup storage again. I have a couple
of questions for you professionals who do this on more of a daily basis.

I have two HGST 4T drives (model HDN724040ALE640) that I will setup in
RAID-1. In addition, I have one of the new WD Red Pro drives on the way.
I'll probably use it as a hot spare.

Questions:

1. Should I worry about partition sizes between various brands? I've
seen this:
  https://raid.wiki.kernel.org/index.php/Partition_types
In the past, I did replace a WD Green with a WD RE4 (quiet Jonathan!),
but the RE4 was just slightly bigger so it was no problem to clone the
partition table. Has this ever been an issue for someone bringing an new
drive that was just a little too small to an existing array? How much
variance is there in 4T capacity across models?

2. Would it make sense to use one of the HGST drives as the spare on the
theory that failure would be less correlated between two drives from
different manufacturers? I'm not really concerned about speed, but I
guess these are all 7200RPM anyway.

3. Suppose I get another 4T drive in the future, is it
possible/reasonable to switch the array to RAID-6 after the fact?
Looking at:
  htts://raid.wiki.kernel.org/index.php/Growing
It seems possible. It isn't clear if this is risky. Actually, I guess
this question also applies to the 2 old 2T drives in this box, which I
could move to my desktop to convert raid-1 -> raid-6.

4. Is it better to move the data from my 2T array to the new 4T array at
the raid level or the lvm level? My initial plan was to create the new
array and use lvm to move the file systems, correct?

5. Is mdadm monitoring enough monitoring? Should I have something
looking at the smartd logs?

6. Man, this is more questions than I thought I had... can anyone say
something about spinning down the spare drive?

-- 
Anthony Carrico

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