Linux System Administration

Slightly more tedious and complex than adding a disk drive to other microcomputer systems.

The steps required to add an additional disk drive to a microcomputer system are somewhat more tedious than those needed for larger systems. Most of the complexity comes from the fact that disks can be shared by distinct operating systems on microcomputers.

The terminology related to disk partitions varies somewhat between UNIX and other microcomputer operating systems. For example, DOS distinguishes between the primary partition (the principal, bootable DOS partition) and other extended partitions on the same hard disk; a UNIX disk partition is simply a separately accessible portion of a disk. DOS allows for a maximum of four physical partitions per disk. Under DOS, a physical disk partition can be further subdivided into multiple parts, known as logical drives. The first step in adding a disk to a microcomputer system is to decide how the drive will be split between DOS and UNIX (if applicable). The fdisk utility is used to create physical disk partitions on microcomputer systems (DOS also provides an fdisk utility). The cfdisk utility, a screen-based version of fdisk, is also available under Linux. The following considerations apply to the myriad of fdisk versions that you may encounter:

  • The conventional wisdom is to use the native version of fdisk to create and operate on the disk partitions for each operating system. In other words, use the DOS version for the DOS partitions, and the UNIX version for the UNIX partitions. In practice, you can often get away with using a different version. Things generally work fine, except when they don't.

  • Keep records of the partition numbers, starting and ending blocks, total sizes, partition type, and other data for each disk partition table as it is displayed by every version of fdisk that you have. An easy way to do this is to print the partition table from each version. If the table is ever damaged—and this does happen from time to time—you will probably need the information to recreate it and recover your data. Having the data from all the versions ensures that you can redefine partitions following the same alignment patterns and requirements as observed by the various operating systems when they created the partitions initially.

Making a New Disk Available to Linux

We'll look at the process of attaching a new SCSI disk to a Linux system in detail. The process would be the same for other disk types (for example, IDE), although the special files used to access the device would be different (for example, /dev/hdb).

After attaching the disk to the system, it should be detected when the system is booted. You can use the dmesg command to display boot messages or check /etc/boot.log if you're using the sysvinit facility:

scsi0 : at 0x0388 irq 10 options CAN_QUEUE=32 ...
scsi0 : Pro Audio Spectrum-16 SCSI
scsi : 1 host.
Detected scsi disk sda at scsi0, id 2, lun 0
scsi : detected 1 SCSI disk total.

If necessary, create the device special files for the disk (needed only when you have lots of disks). For example, this command creates the special files used to access the fifth SCSI disk:

# cd /dev; MAKEDEV sde

Note also that disk ordering happens at boot time, so adding a new SCSI disk with a lower SCSI ID than an existing disk will cause special files to be reassigned—and probably break your /etc/fstab setup.

Assuming we have our special files all in order, we will use fdisk or cfdisk (a screen-oriented version) to divide the disk into partitions (we'll be saving about a third of the disk for a DOS partition). The following commands will start these utilities for the first SCSI disk:

# fdisk /dev/sda
# cfdisk /dev/sda

We'll need the following subcommands:

Action

Subcommand

 
 

fdisk

cfdisk

Create new partition

n

n

Change partition type

t

t

Make partition active/bootable

a

b

Write partition table to disk

w

W

Change size units

u

u

Display partition table

p

N/A

cfdisk is more convenient to use as the partition table is displayed continuously. A cfdisk subcommand always operates on the current partition (highlighted). Thus, in order to create a new partition, move the highlight to the line corresponding to Free Space, and then enter an n. cfdisk will prompt for the partition information:

Primary or logical [pl]: p
Size (in MB): 110

If you'd rather enter the size in a different set of units, use the u subcommand (cycles among MB, sectors and cylinders).

We use the same procedure to create a second partition, and then activate the first partition with the b subcommand. Then, we use the t subcommand to change the partition types of the two partitions. The most commonly needed type codes are 6 for DOS, 82 for a Linux swap partition, and 83 for a regular Linux partition.

Here is the final partition table (output has been simplified): Don't put any hairspaces in between the dashes below, or they will blow up. They don't have to look separated.

cfdisk 0.8 BETA
              Disk Drive: /dev/sda
Heads: 16  Sectors per Track: 63   Cylinders: 1023
Name      Flags   Part Type   FS Type    Size (MB)
/dev/sda1 Boot    Primary     Linux          110.0
/dev/sda2         Primary     DOS             52.5
                  Pri/Log     Free Space       0.5

If you've changed the partition layout of the disk—in other words, done anything other than change the types assigned to the various partitions—reboot the system at this point.

Next, use the mkfs command to create a filesystem on the Linux partition. mkfs has been streamlined in the Linux version and requires little input:

# mkfs -t ext2 /dev/sda1

This command creates an ext2-type filesystem. If you want to customize mkfs's operation, the following options may be useful:

  • -b: Set filesystem block size in bytes (the default is 1024).

  • -f: Set filesystem fragment size in bytes (the default is 1024).

  • -c: Check the disk partition for bad blocks before making the filesystem.

  • -i: Specify bytes/inode value: create one inode for each chunk of this many bytes. The default value of 4096 usually creates more than you'll ever need, but probably isn't worth changing.

  • -m: Specify the percentage of filesystem space to reserve (accessible only by root). The default is 5% (half of what is typical on other UNIX systems). In these days of multigigabyte disks, even this percentage may be worth rethinking.

Once the filesystem is built, run fsck:

# fsck -f -y /dev/sda1

The -f option is necessary to force fsck to run even though the filesystem is clean. The new filesystem may now be mounted and entered into /etc/fstab, which is the subject of the next section.

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