Device File Creation

Before a device file is created, the OS kernel has to support the device in question.  If you are talking about disk or tape devices, this is mostly a matter of having the kernel built to know about the SCSI controller that the disk or tape drive is attached to.  If you are adding a disk or tape drive to an existing controller, you should not have to rebuild the kernel.  For more details on building kernels, see the "Building Kernels" page in this site, or look at the online html documentation from Digital.

Device files are generally created automatically at system boot up time by the /dev/MAKEDEVS script.  If you need to run this script manually, you simply run the script as root and specify what type of device you are installing and what it's SCSI target id is.

For example, to create a disk device for a SCSI disk on controller 0, target 0, and a tz tape drive on target 5:

    # cd /dev
    # ./MAKEDEV rz0
    # ./MAKEDEV tz5

I will explain the tape drive and disk drive names that are created  next.
 

Disk and Tape Drive Naming Conventions:

As with most Unixs, disk and tape device file names can be a little obscure.  But once you know the logic behind them, Digital Unix's names are fairly straight forward.
For disk drives, the name is a combination of the type of type of disk, the SCSI controller the disk is on, and the disk's target and LUN numbers.  This naming convention holds true whether the disk is directly attached, or if it is in a RAID device such as an EMC Symmetrix.  In the case of a RAID array, the "disk" is likely to be a logical device that is presented to the host rather than a physical disk.

Disk name format:
/dev/<device type><logical unit><target/controller number><disk slice>

• <device type> refers to whether the device file is a block or character device.  Character devices are usually refereed to as "raw" devices.  This this the type of device that Sybase typically uses.  Raw devices have a leading "r", block devices have no leading letter.  SCSI disks usually start with "rz" (rrz0 for raw), but  I have also seen "re" (rre for raw) as a valid prefix for disk name.
• <logical unit>  This letter specifies the "logical unit number" or "LUN" of the SCSI disk.  This number is usually 0 for normal disk drives.  But RAID arrays often use different LUNs on their logical disk drives as a way to increase the number of logical disks that can be configured on a single SCSI controller.  LUN numbers range between 0-7.  A LUN of 0 has no additional letter in this position.  Starting with LUN 1, the LUN numbers 1-7 correspond to the letters b-h.
• <target/controller number>  This number is computed by using the formula "(SCSI Bus number * 8) + disk target number"  Where SCSI Buses start at 0, and SCSI target numbers are in the range of 0-7.  Target number 7 is usually reserved for the SCSI controller itself.  The SCSI bus number assigned to a SCSI controller is determined by the entries used in the kernel configuration file when it was built.
• <disk slice letter>  This is a letter from a-h.  A SCSI disk can be partitioned or sliced into 8 pieces, with partition "c" being reserved to represent the entire disk.  There will be disk device files for each of the possible slices on the disk, "a-h", even if a given disk partition is of 0 size and is not in use.  Slice "a" is where the root file system lives on a disk if it is a boot disk.  Slice "b" is the default swap partition on the boot disk.  If the disk is not a system boot disk, you can use these partition letters for other purposes.

For each given physical or logical disk, there will be a total of 16 device files.  One for each block device disk partition, and one for each character (raw) disk partition.

Example:  /dev/rz0a

• "rz" - specifies disk name is the block device for a SCSI disk.
• Because there is no letter between the "rz" and the "0", the LUN for the drive is "0".
• "0" signifies that this disk is target 0 on SCSI bus 0.  Usually SCSI bus 0 is the SCSI controller built into the system motherboard.
• "a" This is the first partition on the disk.

Example 2:    /dev/rrze17e

• "rz0" - specifies that the disk name is for the character (raw) device of a SCSI disk.
• "e"    - specifies that this disk has a LUN of 5.
• "17"   - 17 DIV 8 = 2, remainder 1.  This means that the disk is disk target 1, on SCSI bus number 2. (The third bus)
• "e"    - Disk partition letter e.  "e" is not designated as having any special purpose.

Tape Device names
Tape devices follow a different naming convention than disks.  You cannot tell what the target number and SCSI bus number is for a tape device given just its name.  The device name will also have other modifiers that determine things like whether the tape rewinds after use, and what density and compression settings are used on the tape.

/dev/<device type><device number><density/compression flag>

• <device type>  This will be "rmt" or "nrmt" for tape drives.  When the "rmt" device is used, the tape drive will rewind the tape after use.  When the "nrmt" device is used, the tape will stay positioned in the middle of the tape after use.  This second behavior is used to place multiple file marks on a tape.
• <device number> Tape device numbers start at 0.  They only signify what order the device files were created in, not what SCSI bus they are attached to, or what their target id is.
• <density / compression flag>  Tape drives are capable of writing at multiple byte per inch densities, and some of them can use data compression hardware to compress the data before it is written to the tape.  "h" is typically the high density/ compressing device.  Writing compressed data can result in better tape utilization.  Drive vendors typically quote a 100% additional capacity figure, but real world values typically result in 50% more capacity for mixed data.  Writing to compression enabled devices will typically be faster than writing to non compression devices.  This is because the bottleneck for a tape drive is typically the speed at which the tape spins.  By getting more data per inch of tape, your writes go faster.

Example: /dev/nrmt1h

• "nrmt" this is the non-rewinding device name for the tape drive.
• "1"  This is the second tape drive on the system.
• "h"  The tape drive is using it's highest density bpi setting, and will use hardware data compression on the data as it is sent to the tape.