I'm trying to set up a disk so that it can multiboot either of two
distinct amd64 OpenBSD installations (I'll call them A and B). Reading
the Fine FAQ suggests that a relatively straightforward way to do this
is to put the two OpenBSD installations inside distinct fdisk partitions,
and use the fdisk 'flag' command to select which fdisk partition
(and hence OpenBSD installation) is booted by default. For example,
I could put OpenBSD installation A inside fdisk partition 0, and OpenBSD
installation B inside fdisk partition 1.
[Of course, to make this work I need to ensure that both root filesystems
are close enough to the start of the physical disk that I don't run into
BIOS disk addressing limits.]
My questions concern an extension of this setup: I'd like the two
installations to share a (large) common /home filesystem. Because of BIOS
disk addressing limits, the large /home filesysgtem must be *after* both
the A and the B root filesystems on the physical disk. This suggests a
disk layout like this:
OpenBSD A OS OpenBSD B OS
partitions partitions shared /home partition
|---------------|----------------|----------------------------------|
disk w x y z disk
start end
My question is, what sort of fdisk partition layout do I want here?
One possibility is to use overlapping fdisk partitions:
* fdisk partition 0 = sectors 64 to disk-end
* fdisk partition 1 = sectors x to disk-end (or maybe x+64 to disk-end)
Here the disklabel partitions inside fdisk partition 0 are set up
to contain the OpenBSD A OS filesystems and the shared /home, and
the disklabel partitions inside fdisk partition 1 are set up
to contain the OpenBSD B OS filesystems and the shared /home.
Another possibility is to use non-overlapping fdisk partitions,
and disklabel offsets which extend outside their fdisk "owners", as per
/usr/src/distrib/amd64/common/install.md:
> The offsets used in the disklabel are ABSOLUTE, i.e. relative to the
> start of the disk, NOT the start of the OpenBSD MBR partition.
That is,
* fdisk partition 0 = sectors 64 to w
* fdisk partition 1 = sectors x to y (or maybe x+64 to y)
* fdisk partition 2 = sectors z to disk-end (or maybe z+64 to disk-end)
Here the disklabel partitions inside fdisk partition 0 are set up
to contain the OpenBSD A OS filesystems and the shared /home, with
the latter having disklabel offsets pointing to fdisk partition 2.
Similarly, the disklabel partitions inside fdisk partition 1 are set up
to contain the OpenBSD B OS filesystems and the shared /home, with
the latter having disklabel offsets pointing to fdisk partition 2.
Clearly, both of these layouts require very careful setup of the disk
offsets -- if I miscalculate then all bets are off. But, assuming that
I calculate correctly....
Question: are there reasons why one or the other of these layouts is
preferable? Or is there some other layout which would be better?
Question: are there any unobvious obstacles to making any/all of
the OpenBSD A OS partitions, the OpenBSD B OS partitions, and/or the
shared /home partition, softraid-crypto?
thanks,
--
-- "Jonathan Thornburg [remove -animal to reply]"
<jth...@astro.indiana-zebra.edu>
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
"There was of course no way of knowing whether you were being watched
at any given moment. How often, or on what system, the Thought Police
plugged in on any individual wire was guesswork. It was even conceivable
that they watched everybody all the time." -- George Orwell, "1984"
