An allocation group. A subvolume in a filesystem which maintains it's own track of free blocks and file data (and it's 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 filesystem, each happen 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, filesystems 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 4GB
An AG or allocation group is a subvolume in a filesystem 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 filesystem, each happen in a different AG.
In SMP systems multiple CPUs can write to different AGs, enabling physically concurrent disk operations on a single filesystem.
Filesystems at that time were designed as much to conserve the limited available disk space as to maximize performance.
Mapping sparse files is relatively straightforward for filesystems that use block allocation, although large sparse files still require a large number of block pointers and thus still experience the inefficiencies of multiple layers of indirection.
This log file is one of the metadata files in the filesystem as discussed previously.
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