The third phase of growth is needed periodically to grow a filesystem over group descriptor block boundaries (at multiples of 16 GB for filesystems with 4 KB blocksize). When the last group descriptor block is full, a new block must be added to the end of the table. However, because the table is contiguous at the start of the first group and is normally followed immediately by the block and inode bitmaps and the inode table, the online resize code needs a bit of assistance while the filesystem is unmounted (offline) in order to maintain compatibility with older kernels. Either at mke2fs time, or for existing filesystems with the assistance of the ext2prepare command, a small number of blocks at the end of the group descriptor table are reserved for online growth. The total amount of reserved blocks is a tiny fraction of the total filesystem size, requiring only a few tens to hundreds of kilobytes to grow the filesystem 1024-fold.
For the third phase, it first gets the next reserved group descriptor block and initializes a new group and group descriptor beyond the end of the filesystem, as is done in second phase of growth. Once this is successful, the superblock is locked while reallocating the array that indexes all of the group descriptor blocks to add another entry for the new block. Finally, the superblock totals are updated, the number of groups is increased by one, and the backup superblock and group descriptors are updated.
The online resizing code takes advantage of the journaling features in ext3 to ensure that there is no risk of filesystem corruption if the resize is unexpectedly interrupted. The ext3 journal ensures strict ordering and atomicity of filesystem changes in the event of a crash - either the entire resize phase is committed or none of it is. Because the journal has no rollback mechanism (except by crashing) the resize code is careful to verify all possible failure conditions prior to modifying any part of the filesystem. This ensures that the filesystem remains valid, though slightly smaller, in the event of an error during growth.