Superblock

A superblock is a metadata structure used by many filesystems to store essential information about the filesystem as a whole. It acts as the system’s entry point for mounting and managing the filesystem, summarizing its size, layout, state, and capabilities.

Usually located within the filesystem’s on-disk structure, the superblock contains fields such as a magic number that identifies the filesystem type, the total number of blocks and inodes, the counts of free blocks and free inodes, the block size and inode size, and the location of key metadata like the block group descriptor table. It also stores identifiers such as a UUID and a volume name, along with timestamps for the last mount and last write operations, and mount-related counters. In addition, the superblock carries feature flags that indicate compatible, read-only compatible, and incompatible features, and it specifies how the filesystem should behave in error scenarios.

The superblock is central to mounting, checking, and repairing a filesystem. When a filesystem is mounted, the kernel reads the superblock to understand the filesystem’s layout and capabilities. If inconsistencies are detected, tools such as filesystem checkers use the information in the superblock to verify and repair metadata and layouts.

Many filesystems provide redundancy for the superblock. In practice, a primary superblock is stored at a fixed location near the start of the volume, and multiple backup copies are placed at predetermined locations across the filesystem. If the primary superblock is damaged, a backup can be used to reconstruct the filesystem’s metadata, enabling recovery with appropriate tooling.

Without a valid superblock, a filesystem typically cannot be mounted, and data recovery depends on the availability of backup metadata. The concept of a superblock is most commonly associated with Unix-like file systems such as ext2, ext3, and ext4, among others.