In computing, an extent is a contiguous area of storage reserved for a file in a file system, represented as a range of block numbers, or tracks on count key data devices.
But because the savings are small compared to the amount of stored data (for all file sizes in general) but make up a large portion of the metadata (for large files), the overall benefits in storage efficiency and performance are slight.
Many modern fault-tolerant file systems also do copy-on-write, although that increases fragmentation.
As a similar design, the CP/M file system uses extents as well, but those do not correspond to the definition given above.
IBM OS/360 and successors allocate files in multiples of disk tracks or cylinders.