The layout of a disk can have many variations, depending on the operating system and volume management software. A disk can be sliced into multiple partitions, or a volume can span multiple partitions on multiple disks. The former layout is discussed here, while the latter, which is more appropriately considered a form of RAID, is covered in Section 11.8.

Each partition can be either “raw,” containing no file system, or “cooked,” containing a file system. Raw disk is used where no file system is appropriate. UNIX swap space can use a raw partition, for example, since it uses its own format on disk and does not use a file system. Likewise, some databases use raw disk and format the data to suit their needs. Raw disk can also hold infor- mation needed by disk RAID systems, such as bitmaps indicating which blocks are mirrored and which have changed and need to be mirrored. Similarly, raw disk can contain aminiature database holding RAID configuration information, such as which disks are members of each RAID set. Raw disk use is discussed in Section 11.5.1.

If a partition contains a file system that is bootable—that has a properly installed and configured operating system—then the partition also needs boot information, as described in Section 11.5.2. This information has its own format, because at boot time the system does not have the file-system code loaded and therefore cannot interpret the file-system format. Rather, boot information is usually a sequential series of blocks loaded as an image intomemory. Execution of the image starts at a predefined location, such as the first byte. This image, the bootstrap loader, in turn knows enough about the file-system structure to be able to find and load the kernel and start it executing.

The boot loader can contain more than the instructions for booting a spe- cific operating system. For instance, many systems can be dual-booted, allow- ing us to install multiple operating systems on a single system. How does the system know which one to boot? A boot loader that understands multi- ple file systems and multiple operating systems can occupy the boot space. Once loaded, it can boot one of the operating systems available on the drive. The drive can have multiple partitions, each containing a different type of file system and a different operating system. Note that if the boot loader does not understand a particular file-system format, an operating system stored on that file system is not bootable. This is one of the reasons only some file systems are supported as root file systems for any given operating system.

The root partition selected by the boot loader, which contains the operating-system kernel and sometimes other system files, is mounted at boot time. Other volumes can be automatically mounted at boot or manually mounted later, depending on the operating system. As part of a successful mount operation, the operating system verifies that the device contains a valid file system. It does so by asking the device driver to read the device directory and verifying that the directory has the expected format. If the format is invalid, the partition must have its consistency checked and possibly corrected, either with or without user intervention. Finally, the operating system notes in its in-memory mount table that a file system is mounted, along with the type of the file system. The details of this function depend on the operating system.

Microsoft Windows–based systems mount each volume in a separate name space, denoted by a letter and a colon, as mentioned earlier. To record that a file system is mounted at F:, for example, the operating system places a pointer to the file system in a field of the device structure corresponding to F:. When a process specifies the driver letter, the operating system finds the appropriate file-system pointer and traverses the directory structures on that device to find the specified file or directory. Later versions of Windows can mount a file system at any point within the existing directory structure.

On UNIX, file systems can be mounted at any directory. Mounting is imple- mented by setting a flag in the in-memory copy of the inode for that directory. The flag indicates that the directory is a mount point. A field then points to an entry in the mount table, indicating which device is mounted there. The mount table entry contains a pointer to the superblock of the file system on that device. This scheme enables the operating system to traverse its directory structure, switching seamlessly among file systems of varying types.