Many new operating system features (such as access control lists, mandatory access controls, Posix Capabilities, and hierarchical storage management) require filesystems to be able associate a small amount of custom metadata with files or directories. In order to implement support for access control lists, Andreas Gruenbacher added support for extended attributes to the ext2 filesystems. [7]
Extended attributes as implemented by Andreas Gruenbacher are stored in a single EA block. Since a large number of files will often use the same access control list, as inherited from the directory's default ACL as an optimization, the EA block may be shared by inodes that have identical extended attributes.
While the extended attribute implementation was originally optimized for use to store ACL's, the primary users of extended attributes to date have been the NSA's SELinux system, Samba 4 for storing extended attributes from Windows clients, and the Lustre filesystem.
In order to store larger EAs than a single filesystem block, work is underway to store large EAs in another EA inode referenced from the original inode. This allows many arbitrary-sized EAs to be attached to a single file, within the limitations of the EA interface and what can be done inside a single journal transaction. These EAs could also be accessed as additional file forks/streams, if such an API were added to the Linux kernel.