Partition Scheme

The multiple partitions that NOOBS4IoT divides your SD card into (at least 5) can be quite overwhelming and confusing. This page will try and explain how it all works, and illustrate how NOOBS4IoT differs from the 'traditional' standalone images.

This article is based on an entry from the NOOBS Wiki and was adopted to reflect the behaviour of NOOBS4IoT.

More partition naming theory

For historical reasons (only 4 'slots' in the partition table), hard-drives and SD cards can only have a maximum of 4 primary partitions. To work around that limitation, one of those primary partitions can optionally be an extended partition. An extended partition can contain an unlimited number of logical partitions inside it. Under Linux, the primary partitions are always numbered 1-4 (i.e. seen as /dev/mmcblk0p1 - /dev/mmcblk0p4 on the Pi), and any logical partitions are always numbered 5 and above (i.e. seen as /dev/mmcblk0p5 and above on the Pi).

NOOBS4IoT partitioning

When NOOBS4IoT is first copied to a FAT-format SD card, there's just a single partition taking up all the space on the card, and this is where the files from the NOOBS4IoT zipfile get written to. In tabular form it looks like:

Primary partition	Logical partition	Type	Label	Contents
1		FAT	New Volume	NOOBS4IoT boot files & initramfs

NOOBS4IoT bootup (low-level)

When the Raspberry Pi is powered on with a NOOBS4IoT card inserted, it:

1. Loads and runs bootcode.bin from the FAT-format /dev/mmcblk0p1, exactly as it does for standalone images. (This behaviour is built into the BCM2835's internal firmware on all Pis, and so can't be changed.)
2. bootcode.bin then spots that start.elf is missing, so it loads and runs recovery.elf instead.
3. Running recovery.elf then switches the firmware into "NOOBS4IoT mode" - it uses recovery.img instead of kernel.img, recovery.cmdline instead of cmdline.txt, and it sets the root filesystem to recovery.rfs.
4. recovery.elf then reads recovery.cmdline and loads and runs recovery.img (the Linux kernel), passing it the entire command-line that it read from recovery.cmdline and telling it to load recovery.rfs as the root filesystem (an initramfs containing various scripts and the NOOBS4IoT application).
5. What happens next depends on which 'mode' NOOBS4IoT is operating in...

NOOBS4IoT bootup (setup mode)

If runinstaller is present in the kernel command-line, then this must be the first time NOOBS4IoT has been booted, so it enters 'setup mode'. It then:

1. Automatically shrinks the first (and only) partition /dev/mmcblk0p1, making it just large enough to hold whatever files it contains, and labels it as 'RECOVERY'.
2. Creates a new large empty extended partition /dev/mmcblk0p2, using up the vast majority of the remaining card space.
3. Creates a new small (32MB) ext4-format partition /dev/mmcblk0p5 at the start of the extended partition, and labels it as 'SETTINGS'. This is used to store files telling NOOBS4IoT which OSes are installed (and what partitions they're installed on) and which OS should be loaded by default.
4. Removes runinstaller from recovery.cmdline to prevent this process from being triggered again.

This then changes the partitions to:

Primary partition	Logical partition	Type	Label	Contents
1		FAT	RECOVERY	NOOBS4IoT boot files & initramfs
2		extended		Any logical partitions
	5	ext4	SETTINGS	NOOBS4IoT settings

NOOBS4IoT bootup (recovery mode)

If NOOBS4IoT detects that no Operating Systems have been installed yet, or runinstaller is defined in the recovery.cmdline, NOOBS4IoT enters 'recovery mode'. This starts the OS installation webserver, allowing the user to choose which OS(es) to install.

NOOBS4IoT datafiles

In contrast to the standalone images described earlier (which contain raw partitions), NOOBS4IoT instead uses (compressed) tarballs of the partition contents, along with a bunch of settings files. NOOBS4IoT is responsible for actually creating the partitions on the SD card itself, which means the partitions are always created at the "correct" size in the first place, there's no need to resize them later. And unlike the low-level raw partitions, the tarballs don't store unused disk blocks.

NOOBS4IoT OS installation

For the first example, let's assume that the user is installing just Raspbian. The partitions.json (which can be viewed online here) then specifies which partitions should be created, how big they should be, and which filesystems they should use. In this example it would create a 60MB FAT partition (/dev/mmcblk0p5), format it, and extract the contents of boot.tar.xz to it. As the root partition has want_maximised: true it would then create an ext4 partition (/dev/mmcblk0p6) filling up the entirety of the rest of the extended partition, format it, and extract the contents of root.tar.xz to it. This gives us the full partition layout shown in the table earlier. It then runs the partition_setup.sh script which mounts these new partitions, and edits files (typically just cmdline.txt on the boot partition and /etc/fstab on the root partition) to tell Raspbian which partitions it got installed to. This allows Raspbian to adjust itself to being stored on /dev/mmcblk0p6 and /dev/mmcblk0p7 instead of /dev/mmcblk0p1 and /dev/mmcblk0p2. And finally it updates the settings partition with details of the OS we just installed.

Primary partition	Logical partition	Type	Label	Contents
1		FAT	RECOVERY	NOOBS4IoT boot files & initramfs
2		extended		Any logical partitions
	5	ext4	SETTINGS	NOOBS4IoT settings
	6	FAT	boot	Raspbian boot files
	7	ext4	root	Raspbian root filesystem

If instead we were installing Raspbian and ArchLinux then we might end up with ArchLinux's boot partition as /dev/mmcblk0p6, ArchLinux's root partition as /dev/mmcblk0p7, Raspbian's boot partition as /dev/mmcblk0p8 and Raspbian's root partition as /dev/mmcblk0p9. As both Raspbian's and ArchLinux's partitions.json file specify one of their partitions as want_maximised: true then we'd end up with two small boot partitions and two large-as-possible root partitions. NOOBS4IoT never 'wastes' any space on an SD card.

Primary partition	Logical partition	Type	Label	Contents
1		FAT	RECOVERY	NOOBS4IoT boot files & initramfs
2		extended		Any logical partitions
	5	ext4	SETTINGS	NOOBS4IoT settings
	6	FAT	boot	ArchLinux boot files
	7	ext4	root	ArchLinux root filesystem
	8	FAT	boot1	Raspbian boot files
	9	ext4	root1	Raspbian root filesystem

NOOBS4IoT bootup ('boot mode')

If the recovery.cmdline does not contain runinstaller or if there is a OS installed, NOOBS4IoT will boot into the partition specified in the default_boot_partition file on the settings partition. This then 'reboots' the firmware and loads start.elf from the specified partition (typically /dev/mmcblk0p6 if only one OS is installed) and then proceeds the same as the standalone boot described at the very top of this page - start.elf loads kernel.img and reads cmdline.txt, and then kernel.img uses the command-line that was passed to it to determine which partition the root filesystem is stored on (typically /dev/mmcblk0p7 if only one OS is installed), and loads the rest of the system from there.