Partitions and Mounting

• The layout of a disk can have many variations, depending on the OS. A disk can be sliced into multiple partitions, or a volume can span multiple partitions on multiple disks (RAID).
• Each partition can be either ``raw'', containing no file system (UNIX swap space can use a raw partition), or ``cooked'', containing a file system.
• Multiple OSs can be installed. How does the system know which one to boot?
  • A boot loader that understands multiple file systems and multiple OSs can occupy the boot space.
  • Once loaded, it can boot one of the OSs available on the disk.
• The root partition, which contains the OS 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 OS.
• As part of a successful mount operation, the OS verifies that the device contains a valid file system. If the format is invalid, the partition must have its consistency checked and possibly corrected, either with or without user intervention.
• Finally, the OS notes in its in-memory mount table structure that a file system is mounted, along with the type of the file system.
• Microsoft Windows-based systems mount each volume in a separate name space, denoted by a letter and a colon (F:).
• On UNIX, file systems can be mounted at any directory. Mounting is implemented 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.
• The mount table entry contains a pointer to the superblock of the file system on that device.