The Universal Disk Format (UDF) is the file system used on DVD discs. UDF has a number of built-in features such as allowing larger files, having improved file descriptors, and packet writing that the iso9660 file system cannot easily accommodate. The UDF format is the next step in compact disc technology.
Creating a File System
To create a file system on a disk that has never had a partition table on it or to change the partition table (called repartitioning the disk), you must first create the new partition table. In this section, you begin by learning about the basic structure and workings of the disk as a storage device. This information is fundamental to your understanding of the file system creation process. You then learn to create a partition table by using the fdisk and GNUparted commands. As with all similar Linux commands, each has its own strengths and weaknesses and none is a perfect choice for all situations. In the end, the partition table you create will be the same no matter which command you use. You then learn to create the file system, using commands appropriate for the type of file system you want to create.
The Microsoft version of fdisk creates both a partition table and the bootloader. In Linux, fdisk creates only the partition table. The bootloader is created later by LILO, GRUB, or another bootloader; no bootloader is necessary to create a file system and store data on a disk, just a partition table.
In fact, IDE disks physically installed as something other than /dev/hda (such as /dev/hdc, the secondary master drive) do not have a bootloader written to them; the space where the bootloader code normally resides is likely to be blank. For SCSI disks, the drive designated in the BIOS as the bootable drive has the bootloader written to it.
The Disk as a Storage Device
Because data storage devices are central to the file system, it is important to understand the workings of the most common data storage device: the hard disk drive. Although they work with a different medium, the basic storage functions of floppy disks and removable disk drives are similar to those of the hard disk.
Mechanically, the hard drive is a metal box that encloses disks, also known as platters, which have a magnetic coating on each side. Multiple disks are typically connected to the same spindle and rotated by a motor. The read and write heads for each side of the disk are moved by a second motor to position them over the area of the disk where the data you are looking for is stored. Each platter is organized into cylinders (the default size is 512 bytes) and sectors, and each platter has a head. Each drive has some electronics on a controller card that, along with the disk controller card on the motherboard of the computer, are capable of placing the heads at the correct space to retrieve the data.
The three components, cylinders, heads, and sectors (CHS), are referred to as the drive geometry and are used to identify specific locations on the drive. The CHS information for the drive is detected by the system BIOS and passed on to the operating system.
The first sector of the disk is called the MBR, or Master Boot Record. It is the most important sector on the disk because it contains the bootloader code and the partition table (the table containing pointers to beginning and end of the logical partitions on the disk). The BIOS gets the system's hardware ready, and then executes the bootloader code. The bootloader code and the bootloader program load the kernel and turn over control of the system to the kernel. Then, Linux is on its way to providing us with one of the best operating system experiences in the world.
The MBR sector is 512 bytes long; the first 446 bytes contain the bootloader code. The next 64 bytes contain the partition table, and the final 2 bytes contain a special code (the hexadecimal values of 55 and AA, respectively) that identifies that sector as the MBR. More details about the MBR can be found Chapter 13, "Backing Up."
Creating the Partition Table
Fedora provides several tools to create, examine, and modify the partition table. Because not all the tools we review are likely to be installed on your system (or other system you might be working on for now), this chapter describes making a partition table, using some command-line and graphical tools that Fedora provides.
The partition table has enough room for only four partitions. When the format was first created, it must have been assumed that four would be plenty. Complex, modern systems with very large hard drives make multiple partitions desirable for any number of unique reasons. To get around this problem, one of the four partitions — typically, partition number four — can be used as an extended partition. In other words, in the partition table, it looks like a big partition taking up the rest of the disk. Actually, it is a link to a table that contains the offsets to as many as 63 partitions for IDE disks and 15 for SCSI disks. One extended partition is chained to the next one in this manner.
fdisk CommandThe Linux fdisk command edits the partition table. You must be the super-user (root) before you can run fdisk (also said in Linux shorthand as "run fdisk as root"). Only hard drives (IDE and SCSI) can be accessed with fdisk, and you must use the device name as an argument. USB hard drives are accessed under SCSI emulation and are treated just as if they were SCSI devices. For example, to open fdisk and use it on the first IDE hard drive on the system, you would type this:
# fdisk /dev/hda
and you would see something like this:
# fdisk /dev/hda
The number of cylinders for this disk is set to 4982. There is nothing wrong with that, but this is larger than 1024, and in certain setups could cause problems with software that runs at boot time:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSes (e.g., DOS FDISK or the OS/2 FDISK)
Pressing the m key displays the help screen as follows:
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