Re: 8 GB boot limit

[Nick Holland <nick@holland-consulting.net>]

Per-Olov Sjöholm wrote:
> 
> One change between 3.4 and 3.5 is according to
> http://www.openbsd.org/plus35.html:
> "Major changes to the i386 master boot record, which now uses EDD if
> available to support booting from a partition more than 8GB from the start
> of the disk. The 8GB size limit is still in place".
> 
> Can somebody explain the last "The 8GB size limit is still in place" ?

plusXX.html is a cumulative list of the changes made.  

There were several things that had to be done for >8G booting to take
place:
  * new MBR
  * new PBR (biosboot(8))
  * updated installboot(8))

The new MBR was leaked before the PBR and installboot(8) changes were
done, and a lot of people who had no idea how this all worked assumed
it was "done", and were puzzled by why it "didn't work".  As a result,
we had to make sure all thing regarding the MBR change included a
warning that the 8G limit is still in place.

This was removed by a later entry in plus35.html:
* Major changes to biosboot(8) and installboot(8), supporting 
  EDD (LBA) mode boots and a shift key-triggered CHS fallback mode. 
  For an encore, remove the previous version's 64KB limit on the 
  size of boot(8). 

Booting beyond the 8G point is now possible *ASSUMING YOUR BIOS AND HW
IS CAPABLE*.  Your 486 is still probably limited to 504M, your Pentium
may be stuck anywhere from 2G to 8G, and we've noted a comical number
of brand new, high-end and expensive systems which support >128G HDs,
but with BIOSs that can't actually pull boot code from past the 128G
point (however, I have a cheap, piece of junk board I picked up, where
the CPU fan on the soldered in chip failed after less than 200 hours
of operation, which has NO problem booting from a 4G partition located
at the tail of a 160G hard disk, so this DOES work). 
Short-sightedness is alive and well in the computer industry!  Who
wouldda thunk?

It is probably worth noting that if your BIOS has limitations you
aren't aware of, you could still hurt yourself as described in 
    http://www.openbsd.org/faq/faq14.html#LargeDrive

> I have until now always put my root partition at the beginning of the disk
> and below 8 GB of size to avoid potential problems. I want if possible
> starting with 3.5 have only a / and /var on my disk to take advantage of
> the space.

This is possible now, but as always, still a bad idea.
If the machine is being set up for testing and evaluation, so you can
find out how big you actually need to make partitions, well, fine.  If
you are going into production, an intelligent partitioning plan is
still *highly* recommended, 
    http://www.openbsd.org/faq/faq4.html#SpaceNeeded

> I thought that OpenBSD 3.5 would handle a root partition greater that 8 GB ?
> Can somebody explain this ? Is this old limit just partially fixed ?

Short version: plus35.html needs to be taken as a whole...

Nick.
-- 
http://www.holland-consulting.net