feat: add mdadm pkg

[sergelogvinov]

Add mdadm package to root filesystem.
Which automatically activate healthy softraid.

PR: siderolabs/pkgs#315

Soft raid has to be created before

mdadm --create --verbose --level=1 --metadata=1.2 --raid-devices=2 --homehost=any /dev/md/RAID1Array /dev/sdb /dev/sdc
mkfs.xfs -L data /dev/md/RAID1Array

Result of command blkid will be like

/dev/sdb: UUID="a6a859b8-0d42-090d-8269-e9d55e5875b4" UUID_SUB="788bcff8-8486-5829-815c-7dd634af8a02" LABEL="any:RAID1Array" TYPE="linux_raid_member"
/dev/sdc: UUID="a6a859b8-0d42-090d-8269-e9d55e5875b4" UUID_SUB="5dfb68d1-c5e6-623f-7c83-c02ea4af2e35" LABEL="any:RAID1Array" TYPE="linux_raid_member"
/dev/md127: LABEL="data" UUID="facbfa7a-45a1-4659-ba85-b689a3c1a2b3" TYPE="xfs"

You can add those yaml lines to mount the softraid at boot time

machine:
  disks:
    - device: /dev/md/RAID1Array
      partitions:
        - mountpoint: /var/mnt/extra

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This change is 

[runiq]

Heya, drive-by question: Am I correct that, if this is merged, the mdadm extension won't be neccesary to mount a (healthy) md-raid volume?

[sergelogvinov]

Heya, drive-by question: Am I correct that, if this is merged, the mdadm extension won't be neccesary to mount a (healthy) md-raid volume?

yes, but not for system disk.
I hope someday mdadm tool will be a part of the system. Most CSIs expect that udev/mdadm manage the raid.

[runiq]

Most CSIs expect that udev/mdadm manage the raid.

Yeah, that's what I noticed too. Can't wait for this (and the other dm-raid related PRs) to land. :)

[github-actions]

This PR is stale because it has been open 45 days with no activity.