An extent is a set of one or more contiguous FSBs that define a region in the filesystem for file data or metadata
A file’s inode lists the extents associated with that file
Extents are used for files, directory metadata and extended attributes when the information exceeds the space reserved in the inode
Using extents helps to
minimize the disk space required to store a file's block map
reduce the effects of fragmentation
improve I/O performance by allowing fewer and larger I/O operations
