How does the boot-process assemble the raid?

Question

Hi everyone,
I just switched from Debian and CentOS to archlinux on all my machines and I really love it so far!
Now I feel that I can really customize my OS the way I want.
But I was thinking about one thing that I don't really understand yet, and of what I have
read on this forum I'm certain that someone knows the answer.
On my primary machine I have an mdadm-raid.
And the partitions looks like this:
/dev/sda                          /dev/sdb
Disk1                              Disk2
/dev/sda1                        /dev/sdb1
100mb ext2 (boot)            100mb empty (I did empty partition so that the disks looks the same.)
/dev/sda2  <- RAID0 ->   /dev/sdb2            /dev/md0
2500MB ext3 (root)          2500MB ext3
/dev/sda3  <- RAID0 ->   /dev/sdb3            /dev/md1
2250MB (swap)                2250MB swap
/dev/sda4  <- RAID0 ->   /dev/sdb4            /dev/md2
245150MB ext3 (home)    245150MB ext3
So there was a little tricky to get it installed and assemble the raid at boot, but now it's working like a dream again.
And what I did what except loading mdadm-hook of course I used the kernel parameter: ro root=/dev/md0 md=0,/dev/sda2,/dev/sdb2 a.s.o. to
tell the initrd how to assemble the raid at boot.
But before archlinux i've done this setup in the installer, and after I've installed for example Debian, the system boots correctly and
assemble the raid at boot, but without the kernel-parameters, how does the system know how the raid should be assembled?
I've tried to include the mdadm.conf but with no success, does the initrd contain the raid information is some way?

Answer 1

To obtain that file, run:
abs
This will provide the abs tree of core,extra, and community in /var/abs.
Then you can get the file:
# vim: set ft=sh:
run_hook ()
    input="$(cat /proc/cmdline)"
    mdconfig="/etc/mdadm.conf"
    # for partitionable raid, we need to load md_mod first!
    modprobe md_mod 2>/dev/null
    # if no config file is present create one from command line parameters
    if ! [ -e $mdconfig ]; then
        #Create initial mdadm.conf
        # scan all devices in /proc/partitions
        echo DEVICE partitions > $mdconfig
        for i in $input; do
        case $i in
                # raid
                md=[0-9]*,/*)
                    device="$(/bin/replace -s,/ "$i" "=" "")"
                    array="$(/bin/replace -s/ "$device" "," " devices=")"
                    echo "ARRAY /dev/$array" >> $mdconfig
                    RAID_FOUND=1
                # partitionable raid
                md=d[0-9]*,/*)
                    device="$(/bin/replace -s=d "$i" "md=" "md_")"
                    array="$(/bin/replace -s/ "$device" "," " devices=")"
                    echo "ARRAY /dev/$array" >> $mdconfig
                    RAID_FOUND=1
                # raid UUID
                md=[0-9]*,[0-9,a-z]*)
                    device="$(/bin/replace -s,/ "$i" "=" "")"
                    array="$(/bin/replace -s/ "$device" "," " uuid=")"
                    echo "ARRAY /dev/$array" >> $mdconfig
                    RAID_FOUND=1
                # partitionable raid UUID
                md=d[0-9]*,[0-9,a-z]*)
                    device="$(/bin/replace -s=d "$i" "md=" "md_")"
                    array="$(/bin/replace -s/ "$device" "," " uuid=")"
                    echo "ARRAY /dev/$array" >> $mdconfig
                    RAID_FOUND=1
            esac
        done
    else
        RAID_FOUND=1
    fi
    if [ "$RAID_FOUND" = 1 ]; then
        # assemble everything
        /sbin/mdassemble.static
    fi
The above is what is provided in mine............