Once upon a time, disk drives had clearly defined geometry. Each disk was actually round, and it spun inside the hard drive. The manufacturer divided each disk into tiny sections, called sectors. Each sector had a number, with sector 0 at the beginning of the disk and the sectors numbered sequentially until the end of the disk. Sectors were gathered into rings, or tracks. Stacks of tracks were aggregated into cylinders. Each disk drive had a number of heads—data-reading devices that read information from the disk as the disk spun beneath them. Taken as a whole, sectors, tracks, and cylinders described the disk geometry.

This all seems simple enough, but today you can’t actually count on disk sectors to actually map to anything useful. Over the years, both hard drive manufacturers and operating systems have set and broken limits. This applies to all aspects of machine design, from the 640KB memory limit to the 504MB disk limit. Hard drive manufacturers avoided these limits by tricking the system BIOS and/or the operating system.

If you’re a hard drive manufacturer making a hard drive with 126 sectors per track, but the most popular operating system can accept only 63 sectors per track, you have a problem. The easy solution is to teach your hard drive to lie. If you claim you have half as many sectors per track but twice as many platters, the numbers still add up, and you can still provide unique sector numbers. Every hard drive manufacturer chooses to lie in a slightly different way. The most obvious examples are flash drives (which still report cylinders, sectors, and tracks, even though they’re not round and don’t spin^[7]) and hardware RAID (which reports the same information about several disks as if they were one). If you read about the history of hard drives, you’ll discover all sorts of interesting lies.

By the time disk geometry information reaches the operating system, it has been through one or more translations. Reach into your head, find the button that says “Accept What You’re Told,” and press it as you repeat the following: Disks are divided into sequentially numbered sectors. Partitions fill a number of consecutive sectors. Sectors are grouped into cylinders, based on the number of heads in the drive. Partitions end on cylinder boundaries.

The installer will display your disk’s disklabel. (You can also see the disklabel once the system is installed and running, as discussed in Chapter 8.)

We’ll look at the disk’s physical information first. While the physical information doesn’t usually directly impact the installation, you need to know how to read it if something goes wrong.

1 # /dev/rsd0c:
2 type: SCSI
3 disk: SCSI disk
4 label: DSA2CW120G3
5 duid: adb697598fa0a010
  flags:
6 bytes/sector: 512
  sectors/track: 63
  tracks/cylinder: 255
  sectors/cylinder: 16065
  cylinders: 14593
7 total sectors: 234441648
8 boundstart: 64
9 boundend: 234436545
  drivedata: 0

Except for the device unique identifier (DUID), you cannot change any of these entries without changing the underlying hardware.

The first entry is the device name, /dev/rsd0c 1. The leading /dev means that this is a device node. The rsd0c is the disk name. sd means that this drive uses the sd(4) device driver, and the 0 means that this is the first drive OpenBSD found and attached. (This is usually, but not always, BIOS drive 0.) The leading r means that we’re addressing the disk in raw mode, while the tailing c means that we’re examining disklabel partition c. Disklabel partition c always matches the entire MBR partition containing this disklabel. Almost any disk that isn’t explicitly IDE will probably show up as a SCSI disk.

The type 2 is a general label describing the disk’s physical interface. Any IDE disk will show up as ESDI (Enhanced Small Device Interface), while SCSI, SAS, SATA, and almost every other type of disk has type SCSI.

The disk field 3 shows what sort of disk is attached to this interface. Here, it shows a SCSI disk, but we knew that already from the type.

The label 4 displays the manufacturer’s name and/or the drive model number. In the case of virtualized servers, this shows virtual drive or something similar.

The duid 5 is the DUID for this disk. If you’ve ever managed a system with more than a couple of disks in it, you know how easy it is to confuse disks. The hardware BIOS identifies disks by the physical port they’re attached to. If you need to replace a SATA or SCSI card, and you get the disks mixed up as you rerun cables, you will have a hard time finding your boot drive again. By using the DUID in your system configuration instead of the BIOS-assigned device name, you will always have the same disk used for the same purpose. As noted earlier, the DUID is the one editable field in the top of the disklabel information.

The bytes per sector, sectors per track, tracks per cylinder, and sectors per cylinder 6 all describe the disk’s geometry. These numbers are all lies, but the total number of sectors on the disk 7 is accurate. You also see the first sector you may fill with disklabel partitions 8, and the last sector you may use 9. (You lose a few sectors due to the hard drive’s geometry transformations. Don’t try to hold the hardware accountable. You can’t win that argument.)

The next section displays the disklabel partitions, and you can alter it as needed. Here’s a disklabel from my desktop:

  16 partitions:
  #             1 size         2 offset 3fstype 4[fsize bsize  cpg]
5  a:          2097121               64  4.2BSD      2048 16384    1 # /
6  b:          4698424          2097185    swap
7  c:        312581808                0  unused
   d:          8388576          6795617  4.2BSD      2048 16384    1 # /tmp
   e:         16736864         15184193  4.2BSD      2048 16384    1 # /var
   f:          4194304         31921057  4.2BSD      2048 16384    1 # /usr
   g:          2097152         36115361  4.2BSD      2048 16384    1 # /usr/X11R6
   h:         20971520         38212513  4.2BSD      2048 16384    1 # /usr/local
   i:          4194304         59184033  4.2BSD      2048 16384    1 # /usr/src
   j:          4194304         63378337  4.2BSD      2048 16384    1 # /usr/obj
   k:        245003968         67572641  4.2BSD      2048 16384    1 # /home

This disklabel declares that it has 16 partitions, but lists only 11. The disklabel has space for 16 partitions, but like the MBR partition table, not all of them have space allocated to them. As with most configuration files in Unix-like operating systems, a hash mark (#) indicates the beginning of a comment. The comments here give the headers for the table above.

The first column is the partition letter. A unique letter identifies each disklabel partition. The first partition in our example is a 5, the second is b 6, the third is c 7, and so on.

The size 1 is the number of sectors the drive uses. In this example, partition a fills 2097121 sectors, partition b 4698424 sectors, and partition c 312581808 sectors.

The offset 2 is the number of sectors from the beginning of the MBR partition where the disklabel partition begins. If a disk is bootable, it has a master boot record (MBR) flagging it as such. The MBR record takes the first 63 disk sectors, numbers 0 through 62. The first sector available for a disklabel partition is sector number 63. Partition a begins on sector 64 in order to correctly align with the memory cells in solid-state disks.

Take a look at partition b. It has an offset of 2097185, meaning it starts in sector 2097185. How do we get there? Well, partition a starts in sector 64 and has a size of 2097121. 2097121+64=2097185, or the first free sector after partition a ends. This seems perfectly sensible until you look at partition c. Disklabel partition c is magical. On every disklabel partition, c represents the entire disk. It has an offset of 0 and a size equal to the number of sectors on the disk. You cannot put a filesystem on partition c; it’s there only for reference. Partition d picks up where partition b left off.

The fstype 3 marks the type of filesystem on this partition. OpenBSD filesystems, such as partition a, are labeled as 4.2BSD. (The OpenBSD filesystem is no longer exactly the same as that from BSD 4.2, mind you.) Partition b is swap space.

The next two columns 4 display the fragmentation behavior of the filesystem on this partition. These values are set by the filesystem creation tool when putting the filesystem on the partition, and should not be changed by hand. If you’re curious, read newfs(8) and its related man pages. The fsize is the fragment size for any file fragments on the partition. The b is the size of a block on disk, in bytes. We talk about FFS fragmentation in Chapter 8. All you really need to know at this point is that FFS and FFS2 are both highly fragmentation-resistant, and neither requires any sort of defragmentation process.

The last column shows the number of cylinders per cylinder group. This is almost always 1 for modern disks.

One interesting thing is that the disklabel can be considered a configuration file for formatting a disk. You could save this disklabel to a file, get an identical hard drive, write this label to that new disk, and perfectly duplicate the partitioning of the old disk on the new.

If at any time you feel confused about your partitioning, print out your current disklabel and compare it to how you would like your system to look.