changing to fewer larger disks

changing to fewer larger disks

[Mikael Abrahamsson]

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?

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: changing to fewer larger disks

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 1468 bytes --]

On Mon, 28 Jul 2014 15:51:57 +0200 (CEST) Mikael Abrahamsson
<swmike@swm.pp.se> 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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 828 bytes --]