Re: debian-installer RAID question

[Andy Smith <andy@strugglers.net>]

Hello,

On Sat, Sep 11, 2021 at 06:55:56PM +0200, Felix Natter wrote:
> How does d-i know how the individual HDDs were combined into a
> RAID1?

d-i doesn't as such. In Linux MD, member devices contain metadata to
allow assembly by udev. You can examine the data on an MD member
device like this:

$ sudo mdadm --examine /dev/sda1
/dev/sda1:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : ae2dfc21:c73ccb97:24ca9476:e67c0532
           Name : arran:0  (local to host arran)
  Creation Time : Sat Dec  2 09:29:21 2017
     Raid Level : raid1
   Raid Devices : 2

 Avail Dev Size : 3901440 (1905.00 MiB 1997.54 MB)
     Array Size : 1950720 (1905.00 MiB 1997.54 MB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
   Unused Space : before=1968 sectors, after=0 sectors
          State : clean
    Device UUID : 0bfc69e4:8e62ef0b:b9c16b32:d7cee4f1

Internal Bitmap : 8 sectors from superblock
    Update Time : Wed Sep  8 23:28:20 2021
       Checksum : 3daa61a0 - correct
         Events : 283


   Device Role : Active device 0
   Array State : AA ('A' == active, '.' == missing, 'R' == replacing)

This is how any system with Linux MD support can assemble and
manipulate your arrays given just the devices. Typically udev sees a
block device with MD metadata and calls the mdadm binary to
incrementally assemble it. Once enough devices have been seen the
array is functional.

Cheers,
Andy

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