Re: disk setup question

On Thu, Oct 29, 2020 at 11:08:20AM -0600, Theo de Raadt wrote:
> Aleksander De <aleksander.de@kagyu.pl> wrote:
>
> > Are there any downsides or potential issues which may happen when
> > extending boundaries for OpenBSD partition on >2TB disk while using
> > MBR for booting it at the same time? I need MBR otherwise the machine
> > will not boot. BIOS/RAID controller does not support UEFI.
>
> The BIOS will use the MBR to identify the bootable partition. Then
> it will load the PBR sector as code, jump to it, and then load further
> bootblock code, and kernel after that. It is important to not have the
> root partition in an earlier part of the disk (I'm being intentionally
> vague).
>
> But our bootcode does not care about the partition limits. What happens
> is disklabel looks at the MBR partition limits -- when non-MBR
> partitions (ie disklabel partitions) are being created, usually during
> install. Beyond that, the MBR size is never looked at again.
>
> Where MBR partition limits can matter, is if you go back to disklabel
> again manually to create partitions. It's splitting the disk up. If
> you are not careful, you could create overlaps. This mostly happens to
> people putting multiple operating systems on a machine. Those people
> need to be careful. Everyone else can ignore this.

Thank you very much Otto and Theo.
--
Aleksander