Disk layout for XO-1.5

Mitch Bradley wmb at laptop.org
Tue Jul 28 16:28:52 EDT 2009

david at lang.hm wrote:
> On Tue, 28 Jul 2009, Mitch Bradley wrote:
>> Another important advantage to partitions is that the existence of a 
>> boot partition isolates the firmware from changes in the filesystem 
>> used for the root.
> can you explain this a bit more?

A conventional BIOS boots by reading a small chunk of 16-bit real-mode 
code from the Master Boot Record, which then does something magic, often 
involving chaining to some other chunk of code that is shoehorned into 
some location that is difficult to inspect (e.g. some reserved sectors 
near the beginning of a partition).

OFW instead reads the kernel and initrd files directly from the 
filesystem - there are no hidden chunks of 16-bit code tucked into nooks 
and crannies.

Linux filesystems tend to evolve rapidly.  ext2/3 sprouted numerous 
extensions over the years, some of them backwards compatible and some 
not.  Now we have ext4, which an ext2 reader can't necessarily handle.  
At some point we might want to switch to btrfs, which would require yet 
another reader.  OFW will probably eventually acquire a btrfs reader 
just for diagnostic purposes, but from a stability and ease of migration 
standpoint, it's better to pin down the format of the boot partition, 
thus decoupling the evolution of the OS from the firmware.

UEFI has taken the same approach - a boot partition with a FAT filesystem.

The boot partition is useful not only for the kernel, but also for 
firmware extensions like extra diagnostics, alternate startup graphics, 
alternate fonts, localizations, ...

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