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An extent is defined as one or more adjacent blocks of data within the filesystem. When storage is added to a file on a VxFS filesystem, it is grouped in extents, as opposed to being allocated a block at a time.
Using extents means that disk I/O to and from a file can be done in units of multiple blocks. This type of I/O can occur if storage is allocated in units of consecutive 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 of multiple blocks.