Disk layout for XO-1.5

[david at lang.hm]

On Tue, 28 Jul 2009, Mitch Bradley wrote:

> 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).

I'll point out that lilo allows it to boot from stuff anywhere in the 
filesystem, and does not have to understand the specific filesystem at 
boot time.

there are cases where it doesn't work well, but in the limited case of the 
XO laptops I think it would work well (and avoid the complexity you 
describe below)

Davi dLang

> 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, ...
>