UFS partitioning

Jerry McAllister jerrymc at msu.edu
Fri Dec 5 08:08:01 PST 2008

On Fri, Dec 05, 2008 at 03:58:23AM +1000, Da Rock wrote:

> On Thu, 2008-12-04 at 10:49 -0500, Jerry McAllister wrote:
> > On Thu, Dec 04, 2008 at 12:57:45PM +1000, Da Rock wrote:
> > 
> > > 
> > > On Wed, 2008-12-03 at 20:55 -0500, Robert Huff wrote:
> > > > Da Rock writes:
> > > > 
> > > > >  Excuse my nose in here- I just have a couple of questions.
> > > > >  
> > > > >  1) It IS possible to boot from a dedicated disk?
> > > > 
> > > > 	Yes.  Can't remember the last time I used anything else.
> > > 
> > > So you've never booted from a disk that has been partitioned as a file
> > > system?
> > 
> > You are getting your terms scrambled here.
> > Partitioning has nothing directly to do with creating a file system.
> > You can build a filesystem (with newfs) on just about any piece
> > of disk whether it is the whole disk, a slice of the disk or a partition
> > of a slice.
> > 
> > Making one of those divisions bootable is also pretty much an 
> > independant operation too, though as far as I know, only whole
> > disks and slices can be made bootable but not partitions - the
> > fact that the partition contains the system files is not what
> > makes it bootable.   Being bootable is dependant on the boot sector
> > which gets the control from either the BIOS or an MBR and then finds
> > the system partition (/), mounts it (Read Only) and finds system files 
> > and starts those things running.
> Yes, I would say I'm getting my terms mixed up- fortunately the actual
> reality is clear in my head (hard as that is to believe..).
> I have only one more question then: Why would you use "dangerously
> dedicated mode" at all? I can only see where it might be useful for
> files, no advantage to being a boot sector.

The main reason is because a person doesn't want to bother making
a slice and partitions - so you can get by without it.   

It has no relation to being bootable.  The BIOS hands off control to 
what it thinks is a boot sector and it doesn't matter to it whether 
that is really an MBR that then checks for boot sectors in slices 
and other MBRs on disks or if it is really a boot sector that starts
the reads in the OS files and starts the actual boot.   It is all
the same.

> It was some time ago that I read up on all this, but what I remembered
> was that BSD could use a dedicated disk- but only BSD could read and
> write from it and this is dangerous. Maybe what I was reading was
> regarding bootable and that was considered dangerous... At any rate I'm
> very clear now.

Again, nothing to do with making a disk bootable.  That is determined by
two things:  the presence of the bootable flag; a properly constructed
boot sector to transfer control to.   The 'dangerously dedicated' issue 
is as you say first; BSD can use the dedicated disk both read and write, 
but no one else (other OSen) can - for anything.   Dangerous is probably 
overstating the issue a bit, but became the popular terminology.    

I seem to remember that way back there were some BIOS implementations 
that had trouble with a 'dangerously dedicated' disk and so would not 
boot properly, but the main issue is being able to read/write the disk.


> Thanks for all the information guys- cheers
> _______________________________________________
> freebsd-questions at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-questions
> To unsubscribe, send any mail to "freebsd-questions-unsubscribe at freebsd.org"

More information about the freebsd-questions mailing list