The Superblock

The superblock stores much of the information about the file system. A few of the more important things it contains are:

• Size and status of the file system

• Label (file system name and volume name)

• Size of the file system logical block

• Date and time of the last update

• Cylinder group size

• Number of data blocks in a cylinder group

• Summary data block

• File system state: clean, stable, or active

• Path name of the last mount point

The superblock is located at the beginning of the disk slice, and is replicated in each cylinder group. Because the superblock contains critical data, multiple superblocks are made when the file system is created. Each of the superblock replicas is offset by a different amount from the beginning of its cylinder group. For multiple-platter disk drives, the offsets are calculated so that a superblock appears on each platter of the drive. That way, if the first platter is lost, an alternate superblock can always be retrieved. Except for the leading blocks in the first cylinder group, the leading blocks created by the offsets are used for data storage.

A summary information block is kept with the superblock. It is not replicated, but is grouped with the first superblock, usually in cylinder group 0. The summary block records changes that take place as the file system is used, and lists the number of inodes, directories, fragments, and storage blocks within the file system.