Am 13.09.22 um 21:44 schrieb Wol:

On 13/09/2022 19:03, Reindl Harald wrote:

quoted

Am 13.09.22 um 19:39 schrieb Wols Lists:

quoted

On 13/09/2022 12:12, Reindl Harald wrote:

quoted

"For example, you cannot create 3TB or 4TB partition size (RAID 
based) using the fdisk command. It will not allow you to create a 
partition that is greater than 2TB" makes me nervous

how to get a > 3 TB partition for /dev/md2

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and finally how would the command look for "Then with just two 
drives you change the raid to raid-1"?

the first two drives are ordered to start with 1 out of 4 machines 
ASAP given that the machine in front of me is running since 2011/06 
365/24......

Dare I suggest you read the raid wiki site?

In particular
https://raid.wiki.kernel.org/index.php/Setting_up_a_(new)_system
https://raid.wiki.kernel.org/index.php/Converting_an_existing_system

Also a very good read ...
https://raid.wiki.kernel.org/index.php/System2020
Which is the system I'm typing this on.

These pages don't all jibe with what I remember writing, but read 
them carefully, make sure you understand what is going on, and more 
importantly WHY, and you should be good to go.

And when you're wondering how to go from the 4-drive raid-10 to the 
2-drive raid-1, you should be able to just fail/remove the two small 
drives and everything will migrate to your two new big drives, and 
then it's just whatever the command is to convert between raid 
levels. The drives will already be in a raid-1 layout, so converting 
from 10 to 1 will just be a change of metadata

if it's that easy why aren't mdadm doing it?

[root@testserver:~]$ mdadm /dev/md0 --grow --level=1
mdadm: RAID10 can only be changed to RAID0

virtual machine with two drives replaced against double sized

Hmm...

I don't know. I'll have to defer to the experts, but a raid-10 across 
two drives has to be a plain mirror in order to provide redundancy.

So I don't know why it doesn't just change the array definition, because 
the on-disk layout *should* be the same ...

not really - RAID 10 have stripes and it likely ends in having the data 
on both disk at different places