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

CPU A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary processor in a given computer. Its electronic circuitry executes instructions of a computer program, such as arithmetic, log ...

s can write to different AGs, enabling physically concurrent disk operations on a single file system. Whereas a single

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...

is not capable of reading or writing to multiple places simultaneously, a file system or partition that utilizes a scheme such as

RAID RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical Computer data storage, data storage components into one or more logical units for th ...

logical volume management In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate, ...

may encompass multiple physical disks. In this case, the use of allocation groups may grant additional file system throughput. Allocation groups are used by these file systems: *

XFS XFS is a high-performance 64-bit journaling file system created by Silicon Graphics, Inc (SGI) in 1993. It was the default file system in SGI's IRIX operating system starting with its version 5.3. XFS was ported to the Linux kernel in 2001; a ...

from SGI, an XFS AG can have a max size of 1TiB *

Btrfs Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or "B.T.R.F.S.") is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (distinct from Linux's LVM), d ...

* JFS *

ext2 ext2, or second extended file system, is a file system for the Linux kernel (operating system), kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed ...

ext3 ext3, or third extended filesystem, is a journaling file system, journaled file system that is commonly used with the Linux kernel. It used to be the default file system for many popular Linux distributions but generally has been supplanted by ...

and

ext4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...

use block groups

References

Disk file systems {{Compu-storage-stub