Re: naming system of raid devices

On 02-01-2021 19:18, Wols Lists wrote:

On 02/01/21 15:39, c.buhtz@posteo.jp wrote:

quoted

I do not see the advantage of creating mdadm.conf.
Via fstab I mount the devices by their UUID.
And all other information's mdadm needs to use the RAID is stored in the
superblock.

As the guy who looks after the wiki, none of my systems (to my best
knowledge) have or have had an mdadm.conf. There's supposed to be a way
to create it automagically from the current running config, but I've
never managed to get to grips with it ...

It's easy and in the man page. Several options actually. I typically use:
mdadm --examine --scan > <output file>
I than check the outputfile, if i agree add it to the distro mdadm.conf 
and re-generate the initrd/initramfs or however the distro calls it.

As i often have my boot disk on an array, i want predictability... thus 
i use mdadm.conf in the initrd.

Without mdadm.conf there is no predictability, as order of disk 
detection can, and sometimes indeed does, change.

quoted

So information's in mdadm.conf would be redundant. And especially for
a non-routine home-admin like me each conf-file I modify keep the
possibility of misstakes/missconfigurations and more problems. Keeping
it as simple as possible is very important for my environment.

mdadm.conf is your friend :)

quoted

quoted

Once you are sure it works, I also recommend adding AUTO=-all to
mdadm.conf, so any extra arrays you might plug in temporarily won't
auto-assemble if still plugged in during boot.

This is the start of a philosophical discussion. In other words, 
different minds have different preferences.

quoted

I do not understand this. What does "auto-assemble" means? You mean if
I plug in a SDD/HDD with a mdadm-created superblock?

It's what happens when a system boots - as each drive is recognised,
it's added to an array until the array has enough drives to function. In
other words, rather than doing an mdadm assemble command with all the
drives, it's doing an assemble with just one drive at a time saying "add
to the relevant array".

If you've disabled auto-assemble, presumably you do need mdadm.conf to
tell mdadm what to assemble, which presumably also means you can't have
root on a raid because boot needs the array to find mdadm.conf in order
to start the array ... Don't quote me on that ...

nope. The suggestion here is to define the raids you need in mdadm.conf, 
and then disable auto-creation to prevent surprises from temporarily 
connected disks.

Cheers

Rudy