Which geometry should I use for a disk drive?

Note: By the ``geometry'' of a disk, we mean the number of cylinders, heads and sectors/track on a disk. We will refer to this as C/H/S for convenience. This is how the PC's BIOS works out which area on a disk to read/write from.

This causes a lot of confusion among new system administrators. First of all, the physical geometry of a SCSI drive is totally irrelevant, as FreeBSD works in term of disk blocks. In fact, there is no such thing as ``the'' physical geometry, as the sector density varies across the disk. What manufacturers claim is the ``physical geometry'' is usually the geometry that they have determined wastes the least space. For IDE disks, FreeBSD does work in terms of C/H/S, but all modern drives internally convert this into block references.

All that matters is the logical geometry. This is the answer that the BIOS gets when it asks the drive ``what is your geometry?'' It then uses this geometry to access the disk. As FreeBSD uses the BIOS when booting, it is very important to get this right. In particular, if you have more than one operating system on a disk, they must all agree on the geometry. Otherwise you will have serious problems booting!

For SCSI disks, the geometry to use depends on whether extended translation support is turned on in your controller (this is often referred to as ``support for DOS disks >1GB'' or something similar). If it is turned off, then use N cylinders, 64 heads and 32 sectors/track, where N is the capacity of the disk in MB. For example, a 2GB disk should pretend to have 2048 cylinders, 64 heads and 32 sectors/track.

If it is turned on (it is often supplied this way to get around certain limitations in MSDOS) and the disk capacity is more than 1GB, use M cylinders, 63 sectors per track (not 64), and 255 heads, where 'M' is the disk capacity in MB divided by 7.844238 (!). So our example 2GB drive would have 261 cylinders, 63 sectors per track and 255 heads.

If you are not sure about this, or FreeBSD fails to detect the geometry correctly during installation, the simplest way around this is usually to create a small DOS partition on the disk. The BIOS should then detect the correct geometry, and you can always remove the DOS partition in the partition editor if you do not want to keep it. You might want to leave it around for programming network cards and the like, however.

Alternatively, there is a freely available utility distributed with FreeBSD called pfdisk.exe. You can find it in the tools subdirectory on the FreeBSD CDROM or on the various FreeBSD FTP sites. This program can be used to work out what geometry the other operating systems on the disk are using. You can then enter this geometry in the partition editor.

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