So I’m trying to make some lemonade to go with my crow steak after mistakenly tying Time Machine to Sun’s very cool ZFS. Given that Apple has incorporated Sun’s DTrace into the Darwin kernal, I still have hope they will do the same with ZFS. In my optimistic view, they just haven’t yet. As one of the clearly in-the-know commenters stated

ZFS is not in the Leopard discussed at WWDC in any capacity.

Which is a lot different than saying it will never be in Leopard.

Yet what some of the clearly better-informed-than-me readers say is valuable in and of itself.

What Is The Role Of Journaling?
One reader stated that Time Machine is enabled by journaling. Journaling has been available, in some way shape or form since Mac OS X Server 10.2.2. Yet the modest claims Apple makes for HFS+ journaling don’t square with the reader’s assertion:

Journaling accelerates the recovery time after an unexpected shutdown, significantly improving the availability of server and storage systems. When journaling is turned on on a storage volume, the server automatically tracks file system operations and maintains a continuous record of these transactions in a separate file, called a journal. The operating system can use the journal to return the file system to a known, consistent state after a failure.

In essence then, journaling speeds up the process of consistency checking. There is nothing in journaling that squashes the standard processes of bit rot. There is nothing in journaling per se that enables Time Machine. It isn’t clear that even joining the record of operations with a backup application is all that important. So I don’t get where journaling fits in.

It’s A Backup Application With A Pretty Front End
More likely is this reader’s comment:

It works just like Backup 3. It creates a sparse image and then copies the files onto it in stages. It’s not ZFS or using any special filesystem tricks; it’s just copying files incrementally on a schedule.

This seems to get to the heart of the matter. Time Machine is a pretty front end to a backup application. The presentation focused on finding lost files, and said almost nothing about full restores, but they are implied.

The Limits Of Time Machine
Until I get my hands on it I won’t know for sure, but based on what readers have said I would guess these assertions are likely to be true:

  • You can’t boot off a Time Machine backup
  • If you create and delete a file before the nightly backup Time Machine can’t recover it
  • If your system disk fails you must replace it AND perform the restore – which would take hours
  • Requiring a second drive to find lost files is overkill for laptop users Verdict
I don’t think Time Machine will cause me to change my backup strategy. The fundamental problem of disks going away is a greater concern to me than any one file. That is why I use CarbonCopyCloner to create bootable backups, because if my laptop drive fails I can be back up in minutes, not hours or possibly days – not a lot of SATA notebook drives in small towns.

Requiring a second drive to use Time Machine will be easy for Mac Pro users with multiple drives, but not for my MacBook. Time Machine is a great front end with a weak engine. Let’s hope ZFS is in the works.