On Nov 10, 2017, at 11:04 AM, Phil Turmel [off-list ref] wrote:

On 11/10/2017 01:15 PM, Jun-Kai Teoh wrote:

quoted

I'm backing up as much as I can, but I don't think I'll be able to 
back up everything. I can back up very little of the data, so I'm 
trying to prioritize carefully right now.

Ah, ok.  Well, after you get the important stuff backed up, you can
add more devices as spares and try to let MD continue its grow and
rebuild operations.  It might end up completing.

Again, pardon me if I sound super ignorant, I just want to do all of this right.

Do I just plug in another blank 4TB, and run "mdadm -A /dev/md126 /dev/sd[newdrive]" into it?

Or leave it as it is, and just ask it to continue growing with "mdadm -A -R", and *then* add a blank drive into the raid but don't ask it to grow?

But don't add any more complete devices to the array -- use a partition
that starts at 1MB and covers the rest of the device (default for most
partition tools nowadays).

Where can I read about how to do this properly? I tried googling it but I'm pretty lost. Or what tool I should use etc.

I recommend that when/if your array is stable again, you add one more
spare (with partition) and then use mdadm's --replace operation to move
complete-device members to the new member.  When each is done, take the
newly freed device and partition it and do the next.  When you have no
more complete-device members and have the last freed device partitioned,
consider converting to raid6 with that spare.

I think I understand what you mean here, but it sounds like I need to figure out the top two parts first and that'll take me awhile.

I'll probably have questions again, later, haha.

I really appreciate all the support and patience from this list.

Y'all lifesavers.