On Thu, Jan 06, 2022 at 11:11:30AM -0000, Stuart Henderson wrote:
> You can create more than one "fdisk partition" but there's not much
> point in doing so. It doesn't give you any extra "disklabel partitions".
There is a niche use case for multiple OpenBSD MBR partitions, though:
Imagine a machine with an existing disk layout like this:
Disk: vnd0 geometry: 156250/1/100 [15625000 Sectors]
Offset: 0 Signature: 0xAA55
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
-------------------------------------------------------------------------------
0: 0C 0 0 65 - 41943 0 68 [ 64: 4194304 ] Win95 FAT32L
1: 83 41943 0 69 - 83886 0 72 [ 4194368: 4194304 ] Linux files*
2: 82 83886 0 73 - 125829 0 76 [ 8388672: 4194304 ] Linux swap
3: 00 0 0 0 - 0 0 0 [ 0: 0 ] unused
The spoofed disklabel would look something like:
# /dev/rvnd0c:
type: vnd
disk: vnd device
label: fictitious
duid: 0000000000000000
flags:
bytes/sector: 512
sectors/track: 100
tracks/cylinder: 1
sectors/cylinder: 100
cylinders: 156250
total sectors: 15625000
boundstart: 0
boundend: 15625000
drivedata: 0
16 partitions:
# size offset fstype [fsize bsize cpg]
c: 15625000 0 unused
i: 4194304 64 MSDOS
j: 4194304 4194368 ext2fs
k: 4194304 8388672 unknown
Note that there is about 1.5 Gb of free space at the end of the disk.
If you now wanted to install OpenBSD on this disk, keeping the Linux
installation but deleting the FAT partition at the beginning, you could edit
the MBR to include two OpenBSD MBR partitions:
Disk: vnd0 geometry: 156250/1/100 [15625000 Sectors]
Offset: 0 Signature: 0xAA55
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
-------------------------------------------------------------------------------
0: A6 0 0 65 - 41943 0 68 [ 64: 4194304 ] OpenBSD
1: 83 41943 0 69 - 83886 0 72 [ 4194368: 4194304 ] Linux files*
2: 82 83886 0 73 - 125829 0 76 [ 8388672: 4194304 ] Linux swap
3: A6 125829 0 77 - 156249 0 100 [ 12582976: 3042024 ] OpenBSD
The spoofed disklabel would now look like this:
# /dev/rvnd0c:
type: vnd
disk: vnd device
label: fictitious
duid: 0000000000000000
flags:
bytes/sector: 512
sectors/track: 100
tracks/cylinder: 1
sectors/cylinder: 100
cylinders: 156250
total sectors: 15625000
boundstart: 64
boundend: 4194368
drivedata: 0
16 partitions:
# size offset fstype [fsize bsize cpg]
c: 15625000 0 unused
i: 4194304 4194368 ext2fs
j: 4194304 8388672 unknown
Note that the bounds of the OpenBSD partition no longer cover the whole disk,
as they did when there was no OpenBSD MBR partition, but now only span the
space defined by the first OpenBSD MBR partition.
We can change the bounds:
# /dev/rvnd0c:
type: vnd
disk: vnd device
label: fictitious
duid: a8dacebd60fac195
flags:
bytes/sector: 512
sectors/track: 100
tracks/cylinder: 1
sectors/cylinder: 100
cylinders: 156250
total sectors: 15625000
boundstart: 64
boundend: 15625000
drivedata: 0
16 partitions:
# size offset fstype [fsize bsize cpg]
c: 15625000 0 unused
i: 4194304 4194368 ext2fs
j: 4194304 8388672 unknown
And now create disklabel partitions across the whole of the disk:
# /dev/rvnd0c:
type: vnd
disk: vnd device
label: fictitious
duid: a8dacebd60fac195
flags:
bytes/sector: 512
sectors/track: 100
tracks/cylinder: 1
sectors/cylinder: 100
cylinders: 156250
total sectors: 15625000
boundstart: 64
boundend: 15625000
drivedata: 0
16 partitions:
# size offset fstype [fsize bsize cpg]
a: 4194304 64 4.2BSD 2048 16384 1
b: 1048624 12582976 swap
c: 15625000 0 unused
d: 1993376 13631616 4.2BSD 2048 16384 1
i: 4194304 4194368 ext2fs
j: 4194304 8388672 unknown
Note that the disklabel editor will not, by default, allow you to create a
disklabel partition which overlaps space used by one of the other MBR
partitions, which is usually a good thing and probably the behaviour that most
people would want.
However, if we instead wanted to use the space currently allocated as a Linux
swap partition as our OpenBSD 'b' partition, I.E. share swap space with the
Linux installation on the same disk, and re-allocate the space currently used
as 'b' above to a 4.2BSD partition, 'e', we can, by removing the 'j' partition
from the disklabel entirely:
# /dev/rvnd0c:
type: vnd
disk: vnd device
label: fictitious
duid: a8dacebd60fac195
flags:
bytes/sector: 512
sectors/track: 100
tracks/cylinder: 1
sectors/cylinder: 100
cylinders: 156250
total sectors: 15625000
boundstart: 64
boundend: 15625000
drivedata: 0
16 partitions:
# size offset fstype [fsize bsize cpg]
a: 4194304 64 4.2BSD 2048 16384 1
b: 4194304 8388672 swap
c: 15625000 0 unused
d: 1993376 13631616 4.2BSD 2048 16384 1
e: 1048640 12582976 4.2BSD 2048 16384 1
i: 4194304 4194368 ext2fs
Admittedly, there are usually much better ways to partition disks, but it does
demonstrate not only a use for multiple OpenBSD MBR partitions, but also how
the BSD disklabel really is independent of other partitioning schemes, and can,
(usefully), present a totally different view of the disk. In the above
example, the fdisk partitions still look the same:
Disk: vnd0 geometry: 156250/1/100 [15625000 Sectors]
Offset: 0 Signature: 0xAA55
Starting Ending LBA Info:
#: id C H S - C H S [ start: size ]
-------------------------------------------------------------------------------
0: A6 0 0 65 - 41943 0 68 [ 64: 4194304 ] OpenBSD
1: 83 41943 0 69 - 83886 0 72 [ 4194368: 4194304 ] Linux files*
2: 82 83886 0 73 - 125829 0 76 [ 8388672: 4194304 ] Linux swap
3: A6 125829 0 77 - 156249 0 100 [ 12582976: 3042024 ] OpenBSD
No other operating system needs to know that OpenBSD is happily scribbling over
the Linux swap partition.
