On Mon, 28 Jul 2014 15:51:57 +0200 (CEST) Mikael Abrahamsson
[off-list ref] wrote:

Hi,

I just had a thought, now that we're seeing much larger drives becoming 
available, like 6TB-drives, it would be nice if it was possible to take an 
array consisting of 2TB-drives, replace a few of them with 6TB-drives 
(with want-replace) and then tell MD to restripe the entire array onto 
these fewer larger drives.

Concrete example:

10 bays, currently RAID6 in 9 disks + 1 spare, all disks 2TB ones.

If I took out the spare and replaced it with a 6TB drive and one by one, 
did want-replace so 5 of the drives were 6TB drives, I now potentially 
have the space to keep the same contents on 5 6TB drives intead of 9 2TB 
drives.

If I told mdadm to reduce the number of data drives to 5, would it then 
intelligently choose the 5 largest ones and keep the data area intact (or 
even grow it a little), or what would it do? If this is not the case, 
could this functionality needed be added to the feature request list, 
whereever that list might live?

This has come up before (once I think, maybe twice - many months ago). I
think I have it on a to-do list somewhere.....

  http://neil.brown.name/blog/20110216044002#8

Yes, it would be nice.  No, it hasn't been implemented.

Now, if only we could find someone keen on turning feature requests into
working code....

NeilBrown