[disclaimer: developers who didn’t attend WWDC don’t have copies of the Leopard beta yet, and when they do send me one, I won’t be able to discuss how things actually work, so this is based solely on what Apple has stated on their web site]

When I heard the initial description of Apple’s upcoming Time Machine feature, it sounded like it was similar to NetApp Filer snapshots, or possibly the Windows volume shadow copy feature that’s been announced for Vista (and is already in use in Server 2003). The answers, respectively, are “not really” and “yes and no”.

Quoting:

The first time you attach an external drive to a Mac running Mac OS X Leopard, Time Machine asks if you’d like to back up to that drive.

Right from the start, Time Machine in Mac OS X Leopard makes a complete backup of all the files on your system.

As you make changes, Time Machine only backs up what changes, all the while maintaining a comprehensive layout of your system. That way, Time Machine minimizes the space required on your backup device.

Time Machine will back up every night at midnight, unless you select a different time from this menu.

With Time Machine, you can restore your whole system from any past backups and peruse the past with ease.

The key thing that they all have in common is the creation of copy-on-write snapshots of the data on a volume, at a set schedule. The key feature of NetApp’s version that isn’t available in the other two is that the backup is transparently stored on the same media as the original data. Volume Shadow Copy and Time Machine both require a separate volume to store the full copy and subsequent snapshots, and it must be at least as large as the original (preferably much larger).

NetApp snapshots and VSC have more versatile scheduling; for instance, NetApps have the concept of hourly, daily, and weekly snapshot pools that are managed separately, and both can create snapshots on demand that are managed manually. TM only supports daily snapshots, and they haven’t shown a management interface (“yet”, of course; this is all early-beta stuff).

VSC in its current incarnation is very enterprise-oriented, and it looks like the UI for gaining access to your backups is “less than user-friendly”. I’ve never seen a GUI method of accessing NetApp snapshots, and the direct method is not something I’d like to explain to a typical Windows or Mac user. TM, by contrast, is all about the UI, and may actually succeed in getting the point across about what it does. At the very least, when the family tech-support person gets a call about restoring a deleted file, there’s a chance that he can explain it over the phone.

One thing VSC is designed to do that TM might also do is allow valid backups of databases that never close their files. Apple is providing a TM API, but that may just be for presenting the data in context, not for directly hooking into the system to ensure correct backups.

What does this mean for Mac users? Buy a backup disk that’s much larger than your boot disk, and either explicitly exclude scratch areas from being backed up, or store them on Yet Another External Drive. What does it mean for laptop users? Dunno; besides the obvious need to plug it into something to make backups, they haven’t mentioned an “on-demand” snapshot mechanism, simply the ability to change the time of day when the backup runs. Will you be able to say “whenever I plug in drive X” or “whenever I connect to the corporate network”? I hope so. What does it mean for people who have more than one volume to back up? Not a clue.

Now for the fun. Brian complained that Time Machine is missing something, namely the ability to go into the future, and retrieve copies of files you haven’t made yet. Well, the UI might not make it explicit, but you will be able to do something just as cool: create alternate timelines.

Let’s say that on day X, I created a file named Grandfather, on X+20 a file named Father, and on X+40 a file named Me. On X+55, I delete Grandfather and Father. On X+65, I find myself missing Grandfather and bring him back. On X+70, I find myself longing for a simpler time before Father, and restore my entire system to the state it was in on X+19. There is no Father, there is no Me, only Grandfather. On X+80, I find myself missing Me, and reach back to X+69 to retrieve it.

We’re now living in Grandfather’s time (X+29, effectively) with no trace of Father anywhere on the system. Just Me.

Now for the terror: what happens if you set your clock back?