Alex Tomas and Andreas Dilger implemented support for storing the extended attribute in an expanded ext2 inode, in preference to using a separate filesystem block. In order to do this, the filesystem must be created using an inode size larger than the default 128 bytes. Inode sizes must be a power of two and must be no larger than the filesystem block size, so for a filesystem with a 4 KB blocksize, inode sizes of 256, 512, 1024, 2048, or 4096 bytes are valid. The 2 byte field starting at offset 128 (i_extra_size) of each inode specifies the starting offset for the portion of the inode that can be used for storing EA's. Since the starting offset must be a multiple of 4, and we have not extended the fixed portion of the inode beyond i_extra_size, currently i_extra_size is 4 for all filesystems with expanded inodes. Currently, all of the inode past the initial 132 bytes can be used for storing EAs. If the user attempts to store more EAs than can fit in the expanded inode, the additional EAs will be stored in an external filesystem block.
Using the EA-in-inode, a very large (seven-fold improvement) difference was found in some Samba 4 benchmarks, taking ext3 from last place when compared to XFS, JFS, and Reiserfs3, to being clearly superior to all of the other filesystems for use in Samba 4. [5] The in-inode EA patch started by Alex Tomas and Andreas Dilger was re-worked by Andreas Gruenbacher. And the fact that this feature was such a major speedup for Samba 4, motivated it being integrated into the mainline 2.6.11 kernel very quickly.