Here, you have to resort to some basic math. No, stop screaming; it isn’t that bad: Just get out your calculator. This hard drive has 2,591 cylinders and can hold about 20GB, or 20,000MB. Each cylinder holds roughly the same amount of data. 20,000MB divided by 2591 cylinders equals a little over 7.719MB/cylinder. Dividing the desired partition size in megabytes by the actual MB/cylinder ratio shows that we need 1,036 cylinders for OpenBSD. The first partition goes through partition 891. 891 + 1036 = 1,927, so our OpenBSD partition will end on cylinder 1,927.

Incoming search terms:

• openbsd cylinder boundary

We know that the previous partition ends at the end of cylinder number 891. Our new partition should begin at the beginning of cylinder 892. This would be head 0, sector 1, cylinder 892.

Disk: wd0    geometry: 1 2438/ 2 255/3 63 [ 4 39166470 Sectors]

This line describes what fdisk(8) believes is the disk geometry in the number of cylinders, heads, and sectors a disk has. According to fdisk(8), this disk has 1 2438 cylinders (numbered 0 through 2,437), 255 heads (numbered 0 through 254), and 63 sectors per cylinder. If you compare this information to what the physical label on the hard drive says, it almost certainly won’t match. That’s all right — it’s just been translated. One interesting thing to note is that fdisk(8) reports that this hard drive has the same total number of sectors as every other tool reports, however.

A little beneath that, you get a table describing the MBR partitions themselves.

     Starting     Ending      LBA Info:
#: id  C  H  S -  C  H  S [    start:    size   ]
------------------------------------------------------------------------
* 1 0: 2 0B 3 0 4 1 5 1 - 6 891 7 254 8 63 [   63:   14329917 ] 9 Win95 FAT-32
    1:   00 0  0  0 -   0   0 0 [       0:     0 ] unused
    2:   00 0  0  0 -   0   0 0 [       0:     0 ] unused
    3:   00 0  0  0 -   0   0 0 [       0:     0 ] unused

This isn’t nearly as confusing as it looks at first glance. The first column gives the MBR partition number, between 0 and 3. We then see the 2 Partition ID. This is a unique hex number used to identify the type of file system on the partition. Partition ID 0x0B represents FAT32.

fdisk then prints the 2 cylinder, 3 head, and 5 sector where this partition begins. The first partition on this disk begins on cylinder 0, head 1, sector 1 — the beginning of the disk. ^[1]

The next three columns show the cylinder, head, and sector where this partition ends. Compare these numbers to the total number of cylinders, heads, and sectors in the drive. This disk has 2,438 cylinders, of which we are using 892. Within cylinder number 891, we are using up through head 254 (all of the heads) and  sector 63 (all of the sectors). This Windows partition completely fills the first 2,438 cylinders. We say that such a partition ends on a cylinder boundary. All of your partitions should begin and end on a cylinder boundary.

At the end of the line, we have the partition type in clear English. We could get this information by looking up partition ID 0x0B in a table, but it’s certainly convenient to print it here.

Finally, fdisk presents a command prompt.

fdisk: 1>

We want to create a new MBR partition, immediately following the existing FAT32 partition.

Available disks are: sd0 sd1 wd0.
Which one is the root disk? (or done) [done] wd0
Do you want to use *all* of wd0 for OpenBSD? [no]

We don’t want to use the whole hard drive for OpenBSD, so take the default. This brings up a whole new tool, OpenBSD’s interactive fdisk(8).

Sorry, that’s not the geometry we mean. Disk geometry generally refers to the layout of the disks internally. If you open the case of a hard drive you’ll find a stack of round disks, commonly called platters. They’re covered with a layer of magnetic material that extends from the middle of the disk to the outer edge. When the disk drive is on, these disks spin at thousands of revolutions per minute (rpm). (This is the rpm count you’ll see on the box and in advertising, and has a great deal to do with the performance of an individual disk.)

You will also see a head on each side of each platter. The head moves between the center of the disk and the edge so it can read data from the hard drive beneath it. A fairly typical new hard drive has 16 heads. That’s enough for 8 platters, with a head on each side. So, we can read from 16 different locations on the platters simultaneously, so long as the data you want happens to be on different sides of different platters. Every head has a unique number, starting with 0.

Each platter has a number of circular tracks, or tracks, arranged much like the growth rings in a tree. These tracks hold data as a string of 0s and 1s. A head moves over a particular track at a certain distance from the center of the disk and reads this data as the platter spins by beneath it. When you request data from a different track, the head shifts its position and lets that track rotate past beneath it.

If you stack the particular tracks from all the platters on top of one another, you have a cylinder. For example, the innermost track of each platter forms one cylinder, numbered 0. The next-innermost forms cylinder 1. The 3,022nd track of each platter forms cylinder number 3021. Many operating systems expect to find that MBR partitions encompass complete cylinders and get quite upset if they don’t.

Each track is broken up into segments, called sectors, which can each hold 512 bytes of data. Each sector within a track has a unique number, starting with 1. What’s more, every sector on a hard drive has a unique number. If a particular hard drive has 39,179,952 sectors, you can expect to find each with a number 0 through 39,179,951. Many tools expect to address hard disks by sector numbers. When part of a disk goes bad, smart disks mark the affected sectors and don’t use them.

So, sectors combine into tracks, which are arranged in rings on each platter. Tracks can be stacked into cylinders, and they all combine to make up the hard drive. This all seems simple enough, and it would be, if you could reliably use this information.

Over the years, various limitations have been hit in both hard disk and operating system design. We touched upon the 504MB and 8GB limits. These limits could only be avoided by tricking the system BIOS and/or operating system. If the most popular operating system can only accept 63 sectors per track, but the hard drives your company manufactures have 126 sectors per track, you have a problem — unless, of course, you teach your hard drive to lie. If you claim you have half as many sectors per track, but you have twice as many platters, you can make the problem go away. Everything still adds up to the same number of sectors, after all, and all the tools can still find a unique sector-by-sector number. By the time hard drive information reaches the operating system, it has quite possibly been through one or more of these translations.

When you have only one operating system on a hard drive, this works fine. If your operating system receives or performs a slightly different translation on the disk, however, the translated geometry will not precisely match. The individual sector numbers will still match, but cylinder boundaries may not be the same within the translated geometry. Because many operating systems expect their MBR partitions to begin and end on a cylinder boundary, this is a problem. This is why we use only an operating system’s native tools to create MBR partitions for that OS.

Now that you understand the hardware and the translations it undergoes, let’s look at how to manage these partitions.

Linux can read OpenBSD file systems, if you have a Linux kernel that supports BSD disklabels. Similarly, OpenBSD can read EXT2FS file systems. OpenBSD also recognizes file systems from FreeBSD 4 or earlier, and FreeBSD recognizes OpenBSD file systems. If you want to dual-boot FreeBSD 5 or later with OpenBSD, you need to create your FreeBSD partitions as UFS1. OpenBSD does not support FreeBSD’s UFS2. In any of these combinations, you may have to edit the OpenBSD, Linux, or FreeBSD disklabels to include the sector information for the other operating system partitions to actually be able to mount those partitions, however.

OpenBSD includes fips.exe in the “tools” directory under the release directory. The documentation included with fips.exe is fairly good, and Windows 9x is becoming increasingly rare among the people likely to be installing dual-boot systems, so we aren’t going to go into any detail on how to make it work. Just read the instructions and follow them precicely.

Remember, make your Windows 9x partition no larger than 7.5GB; you want to have enough room to get an OpenBSD root partition on your system!

If you wish to access your Windows files when running OpenBSD, format this Windows file system as FAT32. OpenBSD cannot read NTFS partitions. As you find yourself growing more comfortable with OpenBSD, you will probably find yourself booting into Windows less and less frequently, and being able to access that disk space is nice. I know people who started off with dual-boot systems, but finally converted their Windows partitions into MP3 storage without having to reinstall.

Do not attempt to lay out your OpenBSD partition, or subsequent Windows partitions, with the Windows installer! You will quite possibly confuse OpenBSD, Windows, or both. Similarly, do not attempt to create Windows NT partitions (even FAT32 ones) with the OpenBSD installer. Once you have both Windows and OpenBSD installed, you can go in and create additional Windows logical drives.