An AG or allocation group is a subvolume in a file system which maintains its own track of free blocks and file data (and its own journal, in the case of XFS). This makes simultaneous file operations possible; only one write can happen to an AG at any time, but multiple operations can be performed on the file system, each happening in a different AG. In SMP systems multiple CPUs can write to different AGs, enabling physically concurrent disk operations on a single file system. (Hard disks can only do one thing at a time. However, file systems can span hard disks in various ways (RAID stripe, volume management, etc.)).
Used by these file systems:
XFS from SGI, an XFS AG can have a max size of 1TiB