Future development work in this area involves removing the need to do offline filesystem manipulation to reserve blocks before doing third phase growth. The use of Meta Block Groups [15] allows new groups to be added to the filesystem without the need to allocate contiguous blocks for the group descriptor table. Instead the group descriptor block is kept in the first group that it describes, and a backup is kept in the second and last group for that block. The Meta Block Group support was first introduced in the 2.4.25 kernel (Feb. 2004) so it is reasonable to think that a majority of existing systems could mount a filesystem that started using this when it is introduced.
A more complete description of the online growth is available in [6].