An extent is a contiguous area of storage in a computer file system, reserved for a file. When starting to write to a file, a whole extent is allocated. When writing to the file again, the data continues where the previous write left off. This reduces or eliminates file fragmentation. An extent is presented as an address-length pair, which identifies the starting block address and the length of the extent (in file system or logical blocks). Since Veritas File System (VxFS) is an extent-based file system, addressing is done through extents (which can consist of multiple blocks) rather than in single-block segments. Extents can therefore enhance file system throughput.

Extents allow disk I/O to take place in units of multiple blocks if storage is allocated in contiguous blocks. For sequential I/O, multiple block operations are considerably faster than block-at-a-time operations; almost all disk drives accept I/O operations on multiple blocks.

Extent allocation only slightly alters the interpretation of addressed blocks from the inode structure compared to block-based inodes. A VxFS inode references 10 direct extents, each of which are pairs of starting block addresses and lengths in blocks.

Disk space is allocated in 512-byte sectors to form logical blocks. VxFS supports logical block sizes of 1024, 2048, 4096, and 8192 bytes. The default block size is 1 KB for file system sizes of up to 2 TB, and 8 KB for file system sizes 2 TB or larger.