extent
also: extent mapping, extents
A contiguous block of disk space allocated to a file in a filesystem. Extents group consecutive sectors together to reduce fragmentation and improve I/O efficiency.
An extent is a continuous run of physical disk blocks assigned to store a file's data. Instead of tracking every individual block, modern filesystems like ext4, XFS, and Btrfs use extents to represent large contiguous allocations with a single metadata entry, reducing overhead and fragmentation.
For example, if a 1MB file occupies blocks 1000–1255 on disk, that entire range is represented as one extent rather than 256 separate block pointers. This is more efficient than older block-based approaches, especially for large files.
You can view extent information for a file using filefrag to see how fragmented it is, or debugfs for detailed extent metadata on ext4 filesystems.