Editing XFS (section)

===Extent-based allocation===
Blocks used in files stored on XFS file systems are managed with variable length [[extent (file systems)|extents]] where one extent describes one or more contiguous blocks. This can shorten the list of blocks considerably, compared to file systems that list all blocks used by a file individually.

Block-oriented file systems manage space allocation with one or more block-oriented bitmaps; in XFS, these structures are replaced with an extent oriented structure consisting of a pair of [[B+ tree]]s for each file system allocation group. One of the B+ trees is indexed by the length of the free extents, while the other is indexed by the starting block of the free extents. This dual indexing scheme allows for the highly efficient allocation of free extents for file system operations.