An AG or allocation group is a subvolume in a
file system
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
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 electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
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 platters coated with magnet ...
is not capable of reading or writing to multiple places simultaneously, a file system or partition that utilizes a scheme such as
RAID
Raid, RAID or Raids may refer to:
Attack
* Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground
* Corporate raid, a type of hostile takeover in business
* Panty raid, a prankish raid by male college ...
or
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; as ...
from
SGI SGI may refer to:
Companies
*Saskatchewan Government Insurance
*Scientific Games International, a gambling company
*Silicon Graphics, Inc., a former manufacturer of high-performance computing products
*Silicon Graphics International, formerly Rac ...
, 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 simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...
*
JFS
*
ext2
The ext2 or second extended file system is a file system for the Linux kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed according to the same pr ...
,
ext3
ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on extend ...
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
Disk file systems
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