split the Centos code over 2 different physical media.

1. introduction

The original documentation will guide you to use dd for putting the OS on a microSD card.

The result is a microSD card with 3 partions:

        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1            2048     1026047      512000    c  W95 FAT32 (LBA)
/dev/mmcblk0p2         1026048     2074623      524288   82  Linux swap / Solaris
/dev/mmcblk0p3         2074624    30253055    14089216   83  Linux

However, in this tutorial, we are going to put only the boot and swap partition on a very small microSD card, in our case 2GB.

The root partition will be put on an external (more reliable) media such as an USB key or a SSD disk via the USB bus.

In our scenario, both devices, a microSD and an USB key will be mounted as /dev/sdb.

this procedure will unconditionally re-partition and format your disk(s).

2. prepare

wget http://mirror.centos.org/altarch/7/isos/armhfp/CentOS-Userland-7-armv7hl-Minimal-1708-RaspberryPi3.img.xz
xz -dv CentOS-Userland-7-armv7hl-Minimal-1708-RaspberryPi3.img.xz

kpartx -av CentOS-Userland-7-armv7hl-Minimal-1708-RaspberryPi3.img

add map loop0p1 (253:4): 0 1024000 linear /dev/loop0 2048
add map loop0p2 (253:5): 0 1048576 linear /dev/loop0 1026048
add map loop0p3 (253:6): 0 4194304 linear /dev/loop0 2074624

3. micro SD card

umount the (auto)mounted disk: umount /dev/sdb1

parted -s /dev/sdb mklabel "msdos"
partprobe /dev/sdb
parted --align optimal -s /dev/sdb mkpart primary 0% "512M"
parted --align optimal -s /dev/sdb mkpart primary "512M 100%"
sfdisk --part-type /dev/sdb "1" "0C"
sfdisk --part-type /dev/sdb "2" "82"

mkfs.vfat -n "BOOT" /dev/sdb1
mkswap -L "swap" /dev/sdb2

mkdir -p /mnt/source /mnt/target
mount /dev/mapper/loop0p1 /mnt/source
mount /dev/sdb1 /mnt/target
rsync -aiP /mnt/source/* /mnt/target/

sed -i 's~root=/dev/mmcblk0p3~root=/dev/sda1~' /mnt/target/cmdline.txt

sync; umount /mnt/source/ /mnt/target/

eject

4. external USB key

umount the (auto)mounted disk: umount /dev/sdb1

parted -s /dev/sdb mklabel "msdos"
partprobe /dev/sdb
parted --align optimal -s /dev/sdb mkpart primary "0% 100%"
sfdisk --part-type /dev/sdb "1" "83"

mkfs.ext4 -L "root" -m 1 /dev/sdb1

mkdir -p /mnt/source /mnt/target
mount /dev/mapper/loop0p3 /mnt/source
mount /dev/sdb1 /mnt/target
rsync -aiP /mnt/source/* /mnt/target/

echo "#" > /mnt/target/etc/fstab
echo "LABEL=BOOT /boot vfat noatime 0 0" >> /mnt/target/etc/fstab
echo "LABEL=swap swap swap noatime 0 0" >> /mnt/target/etc/fstab
echo "LABEL=root / ext4 noatime 0 0" >> /mnt/target/etc/fstab

sync; umount /mnt/source/ /mnt/target/

eject

5. cleanup

rm -fr /mnt/source/ /mnt/target/

kpartx -dv CentOS-Userland-7-armv7hl-Minimal-1708-RaspberryPi3.img

del devmap : loop0p3
del devmap : loop0p2
del devmap : loop0p1
loop deleted : /dev/loop0