MacOS – How exactly does the new Mountain Lion filesystem work

I haven't found any documentation for this feature yet, so I don't have any specific answers for your questions about interactions with external Time Machine drives.

However, I did do some playing around with the feature while using my laptop on battery. The first thing I noticed is that local snapshots were not enabled by default – for me, at least. I know that because "entering" Time Machine while my backup drive was detached resulted in the usual error message — until I executed the following from a command line:

sudo tmutil enablelocal

After I did this, a hidden volume called MobileBackups was created (in /Volumes) and I was able to "enter" Time Machine. (Perhaps local snapshots weren't enabled on my machine because I have less than 10% free disk space?)

Unfortunately, the feature seems a little half-baked. I tried the following, with pretty unsatisfactory results:

1. Created a new folder, ~/Documents/Test.
2. Forced a new snapshot by running tmutil snapshot.
3. Verified that a new snapshot was created by looking in /Volumes/MobileBackups/Backups.backupdb/hostname/Latests/drivename/Users/myname/Documents.
4. Deleted the Test folder.
5. Entered Time Machine and browsed the backups/snapshots. The Test folder was not there in any of the backups.

Update: I installed Lion on a second Mac, and without any explicit action on my part, this Mac has a MobileBackups volume. And restoring deleted files from a local snapshot works on this Mac.

A transcript of the “Q&A” session in which Linus made the comment is available, but it seems he wasn't asked to elaborate. I'm not sure whether a more in-depth analysis of his opinion on HFS+ has been written down somewhere else.

For someone else's analysis of the matter, you can take a look at John Siracusa's Mac OS X reviews. In particular the one for Mac OS X Lion which has a section titled “What's wrong with HFS+.” I think the most salient bit is (emphasis mine):

Concurrency, metadata written in the correct byte order, sub-second date precision, support for massive volume sizes, and sparse file support are all common features of Unix file systems. Mac OS X, of course, is built on a Unix foundation. When HFS+ was ported from classic Mac OS to Mac OS X, it needed to be extended to support some minimum set of features that are expected from Unix file systems.

Some of those features were an easy fit, but others were very difficult to add to the file system without breaking backwards compatibility. One particularly scary example is the implementation of hard links on HFS+. To keep track of hard links, HFS+ creates a separate file for each hard link inside a hidden directory at the root level of the volume. Hidden directories are kind of creepy to begin with, but the real scare comes when you remember that Time Machine is implemented using hard links to avoid unnecessary data duplication.

The important point here is that Mac OS X is using a file system which wasn't even designed for a Unix system, it was designed for classic Mac OS and patched to implement the features of Mac OS X 10.0 while maintaining backwards compatibility. Apple has subsequently implemented the additional features that it now has in Mac OS X 10.7 (journaling, metadata, filesystem events ...) using the same patching approach rather than a “design from the ground up” approach. I'm not sure how to explain this non-technically, but you could say that all of these additional features are resting on a classic Mac OS foundation that was never designed to support them. This means the solution isn't as good as it could be. The example that Siracusa goes on to discuss is that the solution Apple had to use for hard links while working within the limitations of HFS+ is too sensitive to hardware failure, which is compounded by the fact that HFS+ was also never designed to concern itself with data integrity. Of course, maintaining compatibility with classic Mac OS was a desirable limitation in Mac OS X 10.0 but it really isn't anymore in Mac OS X 10.7.