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Date: Thu, 27 Apr 1995 20:15:29 -0400
From: Gene Stark <gene@starkhome.cs.sunysb.edu>
Message-Id: <199504280015.UAA16200@starkhome.cs.sunysb.edu>
To: current@FreeBSD.org
Subject: Disk slicing problems
Sender: current-owner@FreeBSD.org
Precedence: bulk

Over the past week or two, I have been trying to get various systems
here bootstrapped over to the new disk slicing code.  I have to say,
I haven't fully understood it yet, and there are some serious problems
I have come up against.  Perhaps somebody (Bruce?) can tell me what I
am missing:

	(1)  Though I can mount, say /dev/wd0s4e, /dev/wd0s4f on
		/usr and /local, I am not yet able to use /dev/wd0s4a
		as / and /dev/wd0s4b as swap.  Is this supported?
		Do I have to do something special in the kernel config,
		like change "root on wd0" to "root on wd0s4"?

	(2)  After having gotten a kernel booted with disk slicing
		turned on, I find that I am unable to modify the MBR,
		and I am also unable to perform certain modifications
		to the disklabel.  My understanding was that I am
		supposed to be able to use /dev/rwd0 as "the whole disk",
		and thus access and modify the MBR that way.  However,
		when I try, say, "fdisk -u /dev/rwd0" it fails to write
		the MBR, complaining that the operation is not supported.

		Also, whereas under older kernels I was able to perform
		various kinds of "dangerous" modifications to the disklabel
		while the system is up, I can no longer do this.
		For example, I had my BSD slice starting at offset 1,
		with the a partition starting at offset 1008.  I wanted
		to shrink the BSD slice so that it starts at offset 1008
		to coincide with the start of the a partition, but there
		seems to be no way to do this once the initial slice
		table and disklabel have been written.  Any attempt to
		change the disklabel in this way results in a complaint
		that the "change would shrink an open partition".
		This even happened while booted from the SCSI drive and
		attempting to modify the wd disklabel.  I had to revert
		to an old kernel with no disk slicing, change the MBR
		and disklabel, then reboot the new kernel.

		In summary, I would like to be able to make these "dangerous"
		modifications, as long as I am willing to accept whatever
		consequences accrue from making them on the fly.  Typically,
		what I would want to do is rewrite the MBR and disklabel,
		then reboot immediately if I moved the root area or something.
		The current system of interlocks seems to effectively
		prevent me from doing anything except reloading my entire
		disk from scratch.

	(3)  I do not understand what the current wisdom is as far as
		laying out BSD partitions with respect to cylinder boundaries.
		With the advent of disk slicing, the disklabel now seems
		to be "relativized" to the beginning of the slice, with
		the effect that it gets very confused about what starts
		at a cylinder boundary and what does not.  I am not
		particularly thrilled about this relativization; I would
		prefer it if the display were made the old way, but if you
		weren't supposed to enter values outside the slice the
		disk drivers simply prevented you from changing the
		disklabel to that.

							- Gene