Some of these updates are clearly hacks, in the sense that if you did not have to worry about backward compatibility, they would certainly not be implemented in that way.
They're not like upgrades from ext2 to ext3, it's like ext2 was modified in a weird way to support new features that older ext2 implementations do not know about.
Hard Links for example are implemented by adding files which look a lot like symblinks. As soon as you create hard links to a file, the file is moved into a invisible folder named "Private Data" and becomes a "node file". The original file is replaced by a stub containing the CNID (special type, special content). That's not as efficient as having iNodes and hard links from day 1. If you mount such a volume on Mac OS Classic, the "Private Data" folder is totally accessible and modifiable by the user, and of course hard links do not work.
Hot file clustering and journaling are implemented in similar ways to hard links (invisible folders).
The opposite example are extended attributes (xattr) which were added in 10.4. A special filesystem structure named the "Attributes File" was reserved in HFS+ from day 1 but never used. De-defragmentation does not rely on any changes to the volume, it's purely done in the Filesystem driver stack.
Things like Copy-on-Write or Snapshotting, are probably impossible to implement in a backward compatible manner though...
> They're not like upgrades from ext2 to ext3, it's like ext2 was modified in a weird way to support new features that older ext2 implementations do not know about.
It's my understand that this is exactly what ext3 is, though! It's designed to look like ext2, but with some extra hidden stuff for journaling...
Yes, in fact, we used to convert ext2 to ext3 with one single command:
tune2fs -j /dev/<device>
This adds a journal to the filesystem, making it ext3. In fact, you could mount ext3 as ext2 (you probably still can). It would just not replay the journal.
The same is pretty much true for ext4, it primarily consists of extension for ext3, but forked as a separate filesystem in order not to destabilise ext3. You can mount an ext2 or ext3 filesystem as ext4. You can mount an ext4 filesystem without extents as ext3.
It should also be noted that ext2 is strongly influenced by UFS1 (FFS), which has been around forever.
ext[234] is not a modern filesystem. However, it is very well understood and stable.
How often can you actually implement new features in an entrenched filesystem and not have to worry about backwards compatibility. This is the case for ext2/3/4, NTFS, and I'm sure many others.
