The process of adding a disk under Linux on the Intel (x86) platform is relatively easy. Assuming you are adding a disk that is of similar type to your existing disks (e.g., adding an IDE disk to a system that already has IDE drives or adding a SCSI disk to a system that already has SCSI drives), the system should automatically detect the new disk at boot time, and all that is left is partitioning it and creating a file system on it. If you are adding a new type of disk (like a SCSI disk on a system that has only IDE drives), you may need to compile in support for your SCSI card to the kernel. (Note that most Linux distributions come with support for many popular SCSI cards as part of the standard installation. If you didn’t recompile the kernel from the original installation, it is unlikely that you will need to recompile the kernel to add support.) To compile in support for a new type of disk, see Linux tutorial 9. Be sure to step through all of the relevant menus and mark the appropriate driver to be compiled either into the base kernel or as a module (assuming it can be compiled as a module).

Once the disk is in place, simply boot the system and you’re ready to go. If you aren’t sure about whether the system can see the new disk, run the dmesg command and see whether the driver loaded and was able to find your disk. For example:

    [root@tedford /root]# dmesg | more

Overview of Partitions

For the sake of clarity, and in case you need to know what a partition is and how it works, let’s do a brief review of this subject. Every disk must be partitioned. Partitions divide up the disk, and each segment acts as a complete disk by itself. Once a partition is filled, it cannot (without special software) automatically overflow onto another partition. Usually, the process of partitioning a disk accomplishes one of two goals: Either the user needs two different operating systems installed and each operating system requires its own partition, or it may be prudent that the usage of space on one partition not interfere with space dedicated to other tasks on other partitions. An example of the latter occurs in user home directories. When users of the system are not the administrators of the system, the administrator must ensure that users don’t consume the entire disk for their personal files. This takes up room needed for logging purposes and temporary files, causing the system to misbehave. To prevent this, a special partition is created for user files so that they don’t overflow into protected system space.

NOTE

It is acceptable to partition a disk so that only one large partition is taking up the entire disk. But beware: If this is the boot partition, the entire partition must fit within the 1024 cylinder boundary, or you may not be able to boot. See Linux tutorial 8 for more information.

Where Disks Exist

Under Linux, each disk is given its own device name. IDE disks start with the name /dev/hdX, where X can range from a through z, with each letter representing a physical device. For example, in an IDE-only system with one hard disk and one CD-ROM, both on the same IDE chain, the hard disk would be /dev/hda, and the CD-ROM would be /dev/hdb. Disk devices are automatically created during system installation. When partitions are created, new devices are used. They take the form of /dev/hdXY, where X is the device letter (as just described), and Y is the partition number. Thus, the first partition on the /dev/hda disk is /dev/hda1, the second partition would be /dev/hda2, and so on. SCSI disks follow the same basic scheme as IDE, except instead of starting with /dev/hd, they start with /dev/sd. Therefore, the first partition on the first SCSI disk would be /dev/sda1, the second partition on the third SCSI disk would be /dev/sdc2, and so on.

Creating Partitions CAUTION

The process of creating partitions is irrevocably destructive to the data already on the disk. Before creating, changing, or removing partitions on any disk, you must be very sure of what’s on the disk being modified, and you need to have a backup if that data is still needed. During the installation process, you probably used a “pretty” tool to create partitions. Unfortunately, Linux platforms don’t ship with a standard utility for creating and managing partitions. A basic mechanism that does exist on all Linux distributions is fdisk. Though it’s small and somewhat awkward, it’s a reliable partitioning tool. Furthermore, in the event you need to troubleshoot a system that has gone really wrong, you should be familiar with basics such as fdisk. The only real downside to fdisk is its lack of a user interface. For this sample run, assume that you want to partition the /dev/hdb device, a 340MB IDE hard disk. (Yes, they do still exist.) You begin by running fdisk with the /dev/hdb parameter:

    [root@tedford /root]# fdisk /dev/hdb

which outputs a simple prompt:

    Command (m for help):

Let’s use m to see what your options are. This menu is reasonably self-explanatory:

    Command (m for help): m
    Command action
    a toggle a bootable flag
    b edit bsd disklabel
    c toggle the dos compatibility flag
    d delete a partition
    l list known partition types
    m print this menu
    n add a new partition
    o create a new empty DOS partition table
    p print the partition table
    q quit without saving changes
    s create a new empty Sun disklabel
    t change a partition's system id
    u change display/entry units
    v verify the partition table
    w write table to disk and exit
    x extra functionality (experts only)
    Command (m for help):

You begin by looking at the existing partition, using the p command (print the partition table):

    Command (m for help): p
    Disk /dev/hdb: 16 heads, 63 sectors, 665 cylinders
    Units = cylinders of 1008 * 512 bytes
    Device Boot Start End Blocks Id System
    /dev/hdb1 * 1 664 334624+ 6 FAT16
    Command (m for help):

You have a little legacy system here, don’t you think? Time to upgrade this disk—start by removing the existing partition using the d command (delete a partition):

    Command (m for help): d
    Partition number (1-4): 1
    Command (m for help):
    And use the p (print the partition table) command to verify the results:
    Command (m for help): p
    Disk /dev/hdb: 16 heads, 63 sectors, 665 cylinders
    Units = cylinders of 1008 * 512 bytes
    Device Boot Start End Blocks Id System
    Command (m for help):

No partition there. Time to start creating partitions. For the sake of discussion, pretend this disk is large enough to accommodate a full workstation configuration. To set this up, you need to create the partitions shown in Image 7-5. So now that you know which partitions to create, let’s do it! Start with the root partition. Given that you have only 340MB to work with, keep root small—only 35MB.

    Command (m for help): n
    Command action
    e extended
    p primary partition (1-4)
    p
    Partition number (1-4): 1
    First cylinder (1-665, default 1): 1
    Last cylinder or +size or +sizeM or +sizeK (1-665, default 665): +35M
    Command (m for help):

Notice the first prompt is for whether you want a primary or extended partition. This is because of a goofy mess created long ago (pre-Linux) when hard disks were so small that no one thought more than four partitions would ever be needed. When disks got bigger and backward-compatibility was an issue, we needed a trick to accommodate more partitions.

The last partition would be an “extended” partition, unseen by the user but able to contain additional partitions. The next question: Which partition number? (You can see the limit of four primary partitions as part of the question.) You start with one, picking the default starting cylinder, and then specify that you want 25MB allocated to it. To create the second partition for swap, type the following:

    Command (m for help): n
    Command action
    e extended
    p primary partition (1-4)
    p
    Partition number (1-4): 2
    First cylinder (68-665, default 68): 68
    Last cylinder or +size or +sizeM or +sizeK (52-665, default 665): +16M
    Command (m for help):

This time, the prompts are identical to those used for the previous partition, with slightly different numbers. However, by default, fdisk is creating ext2 partitions. You need this partition to be of type swap. To do this, use the t (change partition type) command:

    Command (m for help): t
    Partition number (1-4): 2
    Hex code (type L to list codes): L
    0 Empty 16 Hidden FAT16 61 SpeedStor a6 OpenBSD
    1 FAT12 17 Hidden HPFS/NTF 63 GNU HURD or Sys a7 NeXTSTEP
    2 XENIX root 18 AST Windows swa 64 Novell Netware b7 BSDI fs
    3 XENIX usr 24 NEC DOS 65 Novell Netware b8 BSDI swap
    4 FAT16 <32 <N 3c PartitionMagic 70 DiskSecure Mult c1 DRDOS/secFAT-
    5 Extended 40 Venix 80286 75 PC/IX c4 DRDOS/secFAT-
    6 FAT16 41 PPC PReP Boot 80 Old Minix c6 DRDOS/secFAT-
    7 HPFS/NTFS 42 SFS 81 Minix / old Lin c7 Syrinx
    8 AIX 4d QNX4.x 82 Linux swap db CP/M / CTOS .
    9 AIX bootable 4e QNX4.x 2nd part 83 Linux e1 DOS access
    a OS/2 Boot Manag 4f QNX4.x 3rd part 84 OS/2 hidden C: e3 DOS R/O
    b Win95 FAT32 50 OnTrack DM 85 Linux extended e4 SpeedStor
    c Win95 FAT32 (LB 51 OnTrack DM6 Aux 86 NTFS volume set eb BeOS fs
    e Win95 FAT16 (LB 52 CP/M 87 NTFS volume set f1 SpeedStor
    f Win95 Ext'd (LB 53 OnTrack DM6 Aux 93 Amoeba f4 SpeedStor
    10 OPUS 54 OnTrackDM6 94 Amoeba BBT f2 DOS secondary
    11 Hidden FAT12 55 EZ-Drive a0 IBM Thinkpad hi fe LANstep
    12 Compaq diagnost 56 Golden Bow a5 BSD/386 ff BBT
    14 Hidden FAT16 5c Priam Edisk
    Hex code (type L to list codes): 82
    Changed system type of partition 2 to 82 (Linux swap)
    Command (m for help):

The first question is, of course, what partition number do you want to change to? Since you want the second partition to be the swap, you entered 2. The next prompt is a bit more cryptic: the hexadecimal code for the correct partition. Since people don’t remember hex codes very well, the L command lists all available partition types. You spot 82 for Linux Swap, so enter that, and you’re done.

NOTE

It used to be that Linux’s swap partition was limited to 128M. This is no longer the case. You can create a single swap partition as large as 2GB. You can create multiple swap partitions if you like. Now to create /usr. Make it 100MB in size:

Command (m for help): n
    Command action
    e extended
    p primary partition (1-4)
    p
    Partition number (1-4): 3
    First cylinder (85-665, default 85): 85
    Last cylinder or +size or +sizeM or +sizeK (85-665, default 665): +100M
  Command (m for help):

So at this point, you have three partitions: root, swap, and /usr. You can see them by entering the p command:

    Command (m for help): p
    Disk /dev/hdb: 16 heads, 63 sectors, 665 cylinders
    Units = cylinders of 1008 * 512 bytes
    Device Boot Start End Blocks Id System
    /dev/hdb1 1 51 25672+ 83 Linux
    /dev/hdb2 52 84 16632 82 Linux swap
    /dev/hdb3 85 288 102816 83 Linux
    Command (m for help):

Now you need to create the extended partition to accommodate /tmp, /var, and /home. Do so using the n command, just as for any other new partition:

    Command (m for help): n
    Command action
    e extended
    p primary partition (1-4)
    e
    Partition number (1-4): 4
    First cylinder (289-665, default 289): 289
    Last cylinder or +size or +sizeM or +sizeK (289-665, default 665): 665
    Command (m for help):

Instead of designating a megabyte value for the size of this partition, you enter the last cylinder number, thus taking up the remainder of the disk. Let’s see what this looks like:

    Command (m for help): p
    Disk /dev/hdb: 16 heads, 63 sectors, 665 cylinders
    Units = cylinders of 1008 * 512 bytes
    Device Boot Start End Blocks Id System
    /dev/hdb1 1 51 25672+ 83 Linux
    /dev/hdb2 52 84 16632 82 Linux swap
    /dev/hdb3 85 288 102816 83 Linux
    /dev/hdb4 289 665 190008 5 Extended
    Command (m for help):

Now you’re ready to create the last three partitions:

    Command (m for help): n
    First cylinder (289-665, default 289): 289
    Last cylinder or +size or +sizeM or +sizeK (289-665, default 665): +100M
    Command (m for help): n
    First cylinder (493-665, default 493): 493
    Last cylinder or +size or +sizeM or +sizeK (493-665, default 665): +45M
    Command (m for help): n
    First cylinder (585-665, default 585): 585
    Last cylinder or +size or +sizeM or +sizeK (585-665, default 665): 665
    Command (m for help):

Note that for the very last partition, you again specified the last cylinder instead of a megabyte value, so that you are sure that you have allocated the entire disk. One last p command shows you what your partitions now look like:

    Command (m for help): p
    Disk /dev/hdb: 16 heads, 63 sectors, 665 cylinders
    Units = cylinders of 1008 * 512 bytes
    Device Boot Start End Blocks Id System
    /dev/hdb1 1 51 25672+ 83 Linux
    /dev/hdb2 52 84 16632 82 Linux swap
    /dev/hdb3 85 288 102816 83 Linux
    /dev/hdb4 289 665 190008 5 Extended
    /dev/hdb5 289 492 102784+ 83 Linux
    /dev/hdb6 493 584 46336+ 83 Linux
    /dev/hdb7 585 665 40792+ 83 Linux
    Command (m for help):
    Perfect. Commit the changes to disk and quit the fdisk utility by using the w (write table
    to disk and exit) command:
    Command (m for help): w
    The partition table has been altered!
    Calling ioctl() to re-read partition table.