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2. Devices

There is a special nomenclature that linux uses to refer to hard drive partitions that must be understood in order to follow the discussion on the following pages.

In Linux, partitions are represented by device files. These are phoney files located in /dev. Here are a few entries:


brw-rw----    1 root     disk       3,   0 May  5  1998 hda
brw-rw----    1 root     disk       8,   0 May  5  1998 sda
crw-------    1 root     tty        4,  64 May  5  1998 ttyS0

A device file is a file with type c ( for "character" devices, devices that do not use the buffer cache) or b (for "block" devices, which go through the buffer cache). In Linux, all disks are represented as block devices only.

2.1. Device names

2.1.1. Naming Convention

By convention, IDE drives will be given device names /dev/hda to /dev/hdd. Hard Drive A (/dev/hda) is the first drive and Hard Drive C (/dev/hdc) is the third.

Table 2. IDE controller naming convention

drive name drive controller drive number
/dev/hda 1 1
/dev/hdb 1 2
/dev/hdc 2 1
/dev/hdd 2 2

A typical PC has two IDE controllers, each of which can have two drives connected to it. For example, /dev/hda is the first drive (master) on the first IDE controller and /dev/hdd is the second (slave) drive on the second controller (the fourth IDE drive in the computer).

You can write to these devices directly (using cat or dd). However, since these devices represent the entire disk, starting at the first block, you can mistakenly overwrite the master boot record and the partition table, which will render the drive unusable.

Table 3. partition names

drive name drive controller drive number partition type partition number
/dev/hda1 1 1 primary 1
/dev/hda2 1 1 primary 2
/dev/hda3 1 1 primary 3
/dev/hda4 1 1 swap NA
/dev/hdb1 1 2 primary 1
/dev/hdb2 1 2 primary 2
/dev/hdb3 1 2 primary 3
/dev/hdb4 1 2 primary 4

Once a drive has been partitioned, the partitions will represented as numbers on the end of the names. For example, the second partition on the second drive will be /dev/hdb2. The partition type (primary) is listed in the table above for clarity, although the concept is not explained until Section 3.3.

Table 4. SCSI Drives

drive name drive controller drive number partition type partition number
/dev/sda1 1 6 primary 1
/dev/sda2 1 6 primary 2
/dev/sda3 1 6 primary 3

SCSI drives follow a similar pattern; They are represented by 'sd' instead of 'hd'. The first partition of the second SCSI drive would therefore be /dev/sdb1. In the table above, the drive number is arbitraily chosen to be 6 to introduce the idea that SCSI ID numbers do not map onto device names under linux.

2.1.2. Name Assignment

Under (Sun) Solaris and (SGI) IRIX, the device name given to a SCSI drive has some relationship to where you plug it in. Under linux, there is only wailing and gnashing of teeth.

Before


SCSI ID #2        SCSI ID #5       SCSI ID #7        SCSI ID #8
 /dev/sda          /dev/sdb         /dev/sdc          /dev/sdd

After


SCSI ID #2                         SCSI ID #7        SCSI ID #8
 /dev/sda                           /dev/sdb          /dev/sdc

SCSI drives have ID numbers which go from 1 through 15. Lower SCSI ID numbers are assigned lower-order letters. For example, if you have two drives numbered 2 and 5, then #2 will be /dev/sda and #5 will be /dev/sdb. If you remove either, all the higher numbered drives will be renamed the next time you boot up.

If you have two SCSI controllers in your linux box, you will need to examine the output of /bin/dmesg in order to see what name each drive was assigned. If you remove one of two controllers, the remaining controller might have all its drives renamed. Grrr...

There are two work-arounds; both involve using a program to put a label on each partition (see Section 6). The label is persistent even when the device is physically moved. You then refer to the partition directly or indirectly by label.

2.1.3. Logical Partitions

Table 5. Logical Partitions

drive name drive controller drive number partition type partition number
/dev/hdb1 1 2 primary 1
/dev/hdb2 1 2 extended NA
/dev/hda5 1 2 logical 2
/dev/hdb6 1 2 logical 3

The table above illustrates a mysterious jump in the name assignments. This is due to the use of logical partitions (see Section 3.4, which always start with 5, for reasons explained later.

This is all you have to know to deal with linux disk devices. For the sake of completeness, see Kristian's discussion of device numbers below.

2.2. Device numbers

The only important thing with a device file are its major and minor device numbers, which are shown instead of the file size:


$ ls -l /dev/hda

Table 6. Device file attributes

brw-rw---- 1 root disk 3, 0 Jul 18 1994 /dev/hda
permissions   owner group major device number minor device number date device name

When accessing a device file, the major number selects which device driver is being called to perform the input/output operation. This call is being done with the minor number as a parameter and it is entirely up to the driver how the minor number is being interpreted. The driver documentation usually describes how the driver uses minor numbers. For IDE disks, this documentation is in /usr/src/linux/Documentation/ide.txt. For SCSI disks, one would expect such documentation in /usr/src/linux/Documentation/scsi.txt, but it isn't there. One has to look at the driver source to be sure ( /usr/src/linux/driver/scsi/sd.c:184-196). Fortunately, there is Peter Anvin's list of device numbers and names in /usr/src/linux/Documentation/devices.txt; see the entries for block devices, major 3, 22, 33, 34 for IDE and major 8 for SCSI disks. The major and minor numbers are a byte each and that is why the number of partitions per disk is limited.