On Wed, Jul 15, 2015 at 11:20:55AM -0400, Austin S Hemmelgarn wrote:

There really isn't much of a clear distinction between ext3 and ext4 (at
least from an end user standpoint), other than the fact that there are some
options that only the ext4 driver understands (like extent based
allocation).

Yeah, the main reason why we did the ext3 -> ext4 fork was that adding
64-bit numbers required major surgery, and we didn't want to break a
lot of production users who were using ext3.  But from a file system
format perspective, ext2, ext3, and ext4 are the same logical file
system.  There are just multiple different implementations, which all
support slightly different sets of file system features:

   * Linux's ext2
   * Linux's ext3
   * Linux's ext4
   * Hurd's ext2
   * *BSD's ext2
   * Grub's ext2/3/4

The last three implementations are in fact independent ones created
from scratch.   :-)

Fortunately we use the same file system support code, e2fsprogs, for
all of them, which is good since it has a very extensive set of
regression test sets for our fsck program, and we've continued adding
to it as we add new file system features.

					- Ted