Re: naming system of raid devices

Am 01.01.21 um 16:28 schrieb c.buhtz@posteo.jp:

In the last weeks I played around with mdadm - on VMs, Odroid-Devices,
PCs - all with Debian 10.

I observed a (for me) curious behavior about the naming of RAID
devices.

I did "--create /dev/md0" all the time.

But sometimes it results in /dev/md127.
Sometimes it is /dev/md/0.
Sometimes it switches between upgrade of the kernel image (/dev/md127
become /dev/md0 and booting fails).

Googleing this topic brings up a lot of other users discussing about
that problem.

My current solution is to ignore the /dev/*-name and mount the
device/partition by its UUID.

you should do that for about 20 years anyways, no matter RAID or not

Another often reported "solution" is to edit the mdadm.conf. But this
is not a good solution for me. mdadm looks in the superblock and knows
(nearly) everything. Each conf-file I need to edit keeps the
possibility for errors/mistakes/faults (because I am not a sysop/admin
but a simple home-server-wannabe-admin).

but how often is anything changed there?

my mdadm.conf is from 2011 and it's not even the same machine with my 
disks which are in the meantime replaced by SSD

you need to make sure that the current config is part if the initrd 
(dracut -f as example) which happens anyways when the kernel is updated 
and a new initrd is generated for the new kernel

---------------------

[root@srv-rhsoft:~]$ lsinitrd | grep mdadm
-rw-r--r--   1 root     root          315 Aug 16  2014 etc/mdadm.conf

---------------------

[root@srv-rhsoft:~]$ cat /etc/mdadm.conf
MAILADDR root
HOMEHOST localhost.localdomain
AUTO +imsm +1.x -all

ARRAY /dev/md0 level=raid1 num-devices=4 
UUID=1d691642:baed26df:1d197496:4fb00ff8
ARRAY /dev/md1 level=raid10 num-devices=4 
UUID=b7475879:c95d9a47:c5043c02:0c5ae720
ARRAY /dev/md2 level=raid10 num-devices=4 
UUID=ea253255:cb915401:f32794ad:ce0fe396
---------------------

"localhost.localdomain" is intentional, there is a second machine with 
identical UUIDs all over the place cloned by move two disks, resync on 
both and adjust runtime config like networking and so on

My Question is how this names come up? How does mdadm, the kernel or
what ever component is responsible here, decide about the "name" of a
raid device?
And which factors influence the re-nameing of such devices (e.g.
between boot or kernel-updates)?

devices are always a problem because it's unknown when they appear, it's 
even unknown *if* they appear so nothing can wait for them

that's why you should always work with UUID