Not yet complete, probably completely wrong on several important points.

26 years ago · df12151280
--- a/src/etc/etc.i386/INSTALL.linux
+++ b/src/etc/etc.i386/INSTALL.linux
@ -0,0 +1,222 @@
 $OpenBSD: INSTALL.linux,v 1.1 1998/09/20 15:12:06 espie Exp $
 Warning: this document is currently being written. It's not yet complete,
 and probably contains loads of errors.
 	Linux + OpenBSD: it's possible
 	by Marc Espie --  Marc.Espie@openbsd.org
 It is perfectly possible to have linux and openbsd on the same disk.
 As of this writing, OpenBSD can read and write linux' partitions, whereas
 Linux cannot access OpenBSD partitions (they differ from NetBSD partitions).
 You can even install OpenBSD from an ext2fs partition (choose install from
 disk... ext2fs does not appear in the choices, but `default' it is).
 First, make a bootable floppy of linux.  Then, you  have to find some room
 for OpenBSD.  Don't worry about swap space: you can share Linux' swap
 partition.
 If you want to grab space from a Windows/DOS partition, use fips.
 Fips20 knows all about FAT32, so windows 95 is no longer a problem.
 First principles
 ----------------
 OpenBSD does not use the DOS partitions for more than booting.  You just
 need a small DOS partition to put your OpenBSD root.  Afterwards, OpenBSD
 heeds some other information entirely, called the BSD disklabel.  This
 disklabel is another completely distinct description of your hard disk. 
 It does not even have to be consistent with the usual DOS partitions 
 information.
 Throughout this document, we will distinguish between DOS partitions and
 BSD partitions whenever this is necessary.
 For consistency, it's better if all parts of the disks that 
 are actually used by OpenBSD are flagged as occupied, type A6, though it 
 is not necessary.  The only part of the disk that should appear both as a BSD
 partition and as a DOS partition is the root partition: it MUST begin on
 the same sector for the boot process to work.
 One way to visualize things is to picture OpenBSD embedded inside DOS
 partitions: the DOS partition(s) used by OpenBSD may each contain several
 OpenBSD partitions. As long as the DOS partition table has the right
 start and length for each partition it will be kept happy.
 The OpenBSD disklabel is just another mechanism that yield another 
 description of the disk. It is vitally important that the BSD root
 partition start precisely where the corresponding DOS partition is supposed
 to start, and it is better when all BSD partitions stay inside their
 DOS partitions boundary.  Apart from OpenBSD partitions proper, the
 BSD disklabel can yield a BSD description of other DOS partitions, but
 this is not mandatory. If you don't have any constraints, having a correct
 description of all partition is better, but with bigger disks, keep in mind
 that OpenBSD disklabels can't hold more than 16 partitions.
 Contrarily to popular belief, OpenBSD does *NOT* need one contiguous chunk 
 of the disk.  It is probably the simplest setup, but other considerations
 (such as the need to boot several OS, and have several small primary
 partitions that all start before cylinder 1024) may lead you to use
 two chunks for OpenBSD.
 If you can, it is MUCH better to devote a full disk to OpenBSD: this limits
 the number of mistakes you can do. Admittedly there are some cases where
 this isn't a option (my machine is a laptop... I have to cope with the
 harddisk I have).
 Mapping your disk
 -----------------
 Starting from Linux, get a grasp of your partitions. Use df to check which
 is what, then fdisk to get the actual setup of the disk.
 Here is my disk:
 --
 Disk /dev/hda: 128 heads, 63 sectors, 993 cylinders
 Units = cylinders of 8064 * 512 bytes
   Device Boot   Begin    Start      End   Blocks   Id  System
 	/dev/hda1            1        1      260  1048288+   6  DOS 16-bit >=32M
 	/dev/hda2          261      261      273    52416   83  Linux native
 	/dev/hda4          287      287      993  2850624    5  Extended
 	/dev/hda5          287      287      303    68512   82  Linux swap
 	/dev/hda6          304      304      456   616864+  83  Linux native
 	/dev/hda7          457      457      520   258016+  83  Linux native
 	/dev/hda8          521      521      537    68512+  83  Linux native
 	/dev/hda9          538      538      601   258016+  83  Linux native
 (In case you're wondering, yes this is a big disk. The linux playground is 
 large, the OpenBSD area will be huge. As a developper, I usually have loads
 of source & binaries lying around... a simple OpenBSD installation can fit
 within 300 Mb with room to spare).
 Get the display to sectors with u, and jot down the corresponding
 information as well:
 --
 Disk /dev/hda: 128 heads, 63 sectors, 993 cylinders
 Units = sectors of 1 * 512 bytes
   Device Boot   Begin    Start      End   Blocks   Id  System
 	/dev/hda1           63       63  2096639  1048288+   6  DOS 16-bit >=32M
 	/dev/hda2      2096640  2096640  2201471    52416   83  Linux native
 	/dev/hda4      2306304  2306304  8007551  2850624    5  Extended
 	/dev/hda5      2306368  2306368  2443391    68512   82  Linux swap
 	/dev/hda6      2443455  2443455  3677183   616864+  83  Linux native
 	/dev/hda7      3677247  3677247  4193279   258016+  83  Linux native
 	/dev/hda8      4193343  4193343  4330367    68512+  83  Linux native
 	/dev/hda9      4330431  4330431  4846463   258016+  83  Linux native
 Okay, finally switch to expert mode, and note the corresponding data.
 Disk /dev/hda: 128 heads, 63 sectors, 993 cylinders
 Nr AF  Hd Sec  Cyl  Hd Sec  Cyl   Start    Size ID
 1 00   1   1    0 127  63  259      63 2096577 06
 2 00   0   1  260 127  63  272 2096640  104832 83
 3 00   0   0    0   0   0    0       0       0 00
 4 00   0   1  286 127  63  992 2306304 5701248 05
 5 00   1   2  286 127  63  302      64  137024 82
 6 00   1   1  303 127  63  455      63 1233729 83
 7 00   1   1  456 127  63  519      63  516033 83
 8 00   1   1  520 127  63  536      63  137025 83
 9 00   1   1  537 127  63  600      63  516033 83
 Note that this is STILL the same data. The good point about this last
 display is that it is what you're going to see in OpenBSD fdisk !
 You will notice that the 3rd primary partition is empty... this is where 
 I intend to stick my OpenBSD root partition (both DOS and BSD partitions),
 and that I have cylinders 602-993 empty... this is where I intend to
 stick the rest of OpenBSD.
 Before starting to install OpenBSD, now would be a good time to check the
 INSTALL.pt document...  Especially note the alignment restriction of 
 partitions (first sector of a partition must be at head 0, sector 1 of a
 cylinder). This is enforced by linux' fdisk.
 The other point to note is that extended partitions are actually linked
 lists. This will also show up in OpenBSD' fdisk.
 Your clock and OpenBSD
 ----------------------
 One last thing you can do before starting OpenBSD: OpenBSD expects your
 hardware clock to be in universal time, and uses timezones to give you
 local time. With linux, this depends... most distributions use a small
 program called hwclock to set up the system time from the hardware clock
 when booting... there is a --utc option if your hardware clock is in
 universal time, but this is not always the case.
 Check your distribution, on redhat 5.1, this occurs in /etc/rc.d/rc.sysinit
 which loads /etc/sysconfig/clock which defines a variable called UTC before
 calling hwclock.
 - ensure UTC is set to true
 - adjust your hardware clock from the system time if necessary, e.g.,
 hwclock --systohc --utc
 Linux fs and OpenBSD
 --------------------
 There is a problem with many linux rc that do mount all filesystems even
 in single-user mode. After you've installed OpenBSD, if your linux kernel
 knows about bsd disklabels, it may insert lots of bsd partitions in its
 list. Then at the next reboot, you're in trouble. The simplest way around
 that is probably to make sure you have a linux kernel around that doesn't
 know about disklabels.   Otherwise, you may wish to check your inittab
 and your rc to make deadly sure that single-user boot will work.
 The OpenBSD installation
 ------------------------
 If you've got the space, you can install from your ext2fs partitions. This
 is what I did, as I have a slip connexion to the rest of the world, and
 the OpenBSD install floppy does not include slip.
 REMEMBER TO BACKUP ALL IMPORTANT DATA ON YOUR DISK BEFORE DOING THE 
 INSTALLATION !!!
 So you cp floppy*.fs /dev/fd0, then reboot...
 <<<* TO BE COMPLETED *>>>
 Booting
 -------
 First time I booted my system back, I did not get into linux... I forgot
 I had installed lilo in the master boot block, and lilo does not heed
 the active partition flag. Fix was rather simple though. Once I was into
 linux, I just had to edit lilo.conf to read:
 boot=/dev/hda
 map=/boot/map
 install=/boot/boot.b
 prompt
 timeout=50
 image=/boot/myvmlinuz
 	label=linux
 	root=/dev/hda2
 	vga=4
 	read-only
 image=/boot/vmlinuz-2.0.34-1
 	label=redhat
 	root=/dev/hda2
 	read-only
 other=/dev/hda1
 	label=dos
 	table=/dev/hda
 other=/dev/hda3
 	label=obsd
 	table=/dev/hda
 (changed were the last 3 lines) and rerun lilo, and voila, 
 OpenBSD was able to boot !
 Linux and OpenBSD partitions
 ----------------------------
 As of 2.0.35/2.1.122, linux does not support OpenBSD partitions. The way
 the linux kernel works is reasonably straightforward, code in
 drivers/block/genhd.c: when it detects a BSD disklabel, it does read it
 and instantiates some new partitions, that will show up in that drive's
 partition list. Unfortunately, that check is hardcoded for type A5
 (NetBSD/FreeBSD) disklabel, and it recognizes only 8 partitions disklabels
 anyway.
 JUST HACKING THAT CODE TO RECOGNIZE A6 IS NOT ENOUGH !!!   
 MORE IS INVOLVED AS THE OPENBSD PARTITION TABLE FORMAT IS *DIFFERENT*.