[GLLUG] Link two RAIDs in one LVM?

[Andy Smith]

Hi Axel,

On Sat, May 09, 2020 at 01:03:37PM +0200, Dr. Axel Stammler via GLLUG wrote:
> My original (2016) setup included two hard disk drives of not 4 TB
> but 8 TB capacity in a RAID-1 that has reached 92 per cent
> capacity.

[…]

> I have ordered two more […] my first idea was to create a new
> RAID-1 and to combine the two resulting systems via Logical Volume
> Management. What do you think?

How do you intend to combine them? You won't be able to put your
existing array into the LVM without destroying its contents.

You could do a thing where you:

1) set up the new array as the boot device

2) copy OS and data over to it from the original array

3) boot into this and confirm everything works, if necessary
   physically removing the old array to be sure that everything
   works and all your data is there

4) Nuke the old array, recreate it and add it to your LVM. You now
   have one big LVM.

Downside of this is that because only two devices existed at first,
all your data is still on two devices. The other two are largely
empty, so performance-wise all reads will come from only two of your
four devices. As you (re)write things over time they will spread out
if you have set your LVM allocation method to stripe.

If you have good backups then you could of course nuke everything
and put it back straight onto your LVM of 2x RAID-1.

Cheers,
Andy

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