Allocation group

An AG or allocation group is a subvolume in a file system which maintains its own track of free blocks and file data.

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
• BTRFS
• JFS
• ext2, ext3 and ext4 use block groups