Apple collects secrets like a pack rat collects shiny things. It's part of the company's culture. So when someone breaks the code of silence, it sets virtual seismographic needles scratching. That happened this week, when Sun Microsystems CEO Jonathan Schwartz said Apple's upcoming Mac OS X 10.5 Leopard, would rely on a file system that engineers at his company have spent years creating: ZFS.

Just what is ZFS, and why did it send Mac enthusiasts spinning? Read on:

What does ZFS stand for? At one time, it was an acronym for Zettabyte File System, but Sun now prefers that the name stand on its own. "Zetta," by the way, is one of the standard SI prefixes - as are the much more familiar "kilo," "mega" and "giga" - and represents 1,021, or 100,000,000,000,000,000,000. According to Sun, "The largest SI prefix we liked was 'zetta.'" Sun was obviously seeking to evoke a really big number to remind everyone of the file system's data (and number of files) capacity. Sort of like Super Size Me - the movie - but bigger. Way bigger.

Got it. Now what is it? ZFS is a 128-bit file system that Sun announced in 2004 but didn't integrate with its Solaris operating system until 2006. Among ZFS' selling points is huge capacity, storage pooling, fast data snapshots and copy-on-write. As a 128-bit file system, it can store 18 billion billion times more data than current 64-bit systems, such as NTFS, which is Microsoft's file system for Windows. According to calculations on Wikipedia, it would take about 9,000 years to max out ZFS' file limit if 1,000 files were created every second. Pooling eliminates partitions, and much of the hassle with storage, such as figuring which "volume" to stick files on, or how to manage a new external drive.

"You don't have to worry about the details of what's going on with your disks, your storage or your file systems," Jeff Bonwick, chief architect of ZFS, said when Sun rolled out the file system in 2004. "You add disks to your storage pool, file systems consume space automatically as they need it, and administrators don't have to get involved." And copy-on-write, which copies modified data to a new block rather than overwriting existing data, is pertinent here because it's one of the most-common methods used to take quick "snapshots" of a disk (or in the case of ZFS, the storage pool) as point-in-time backups.

Okay. So why am I hearing about ZFS now? Blame Schwartz, who at one point during a new product introduction on Wednesday spilled what sounded like a secret. "This week, you'll see that Apple is announcing at their Worldwide Developers Conference that ZFS has become the file system in Mac OS X," he said. Most alert listeners keyed on the word "the," and heard it as in "the default" or "the only." Swap out the Mac's 22-year-old Hierarchical File System? Between now and October? Wow. The supposition seemed confirmed when a second exec, Marc Hamilton, Sun's director of technology for global education and research, wrote on his blog that "Jonathan noted that Apple will announce this week that the ZFS file system from OpenSolaris will become Apple's new default file system [emphasis mine]."

What's in it for me? Rumblings immediately after Apple's 2006 WWDC pinned ZFS to Time Machine, one of the most touted features of the upcoming Leopard operating system. Time Machine and ZFS seemed a marriage made in heaven, what with ZFS' snapshot skills. Storage-centric bloggers like Robin Harris, who writes StorageMojo, and others, like John Siracusa of Ars Technica, laid out the case for a connection last August. A new file system that drove Time Machine - arguably the coolest made-public feature in Leopard - while eliminating data corruption and the volume concept would be a very good thing, even if it was under the hood and therefore hidden from most users. Bottom line: Mac OS X would make another major technological stride, and blow by Microsoft and its Windows yet again. Xerox this, Redmond!

Sounds great? Where do I get it? Actually, ZFS is already in Leopard, according to reports of recent test builds and posted screenshots. The new file system, however, can't be used on the root partition, meaning it cannot be used as the file system for a Mac's boot drive, such as the hard disk inside a MacBook Pro. That's a limitation of ZFS, by the way, not Leopard. Solaris, in fact, has only recently managed to boot its own Solaris operating system from ZFS. The thinking, of course, is that by the time October rolls around and Leopard goes final, the ZFS boot problem, and others, will have been solved by Apple engineers, who are presumably hard at work in an undisclosed location.

I hear a "But ..." coming And you would be right. Almost as soon as ZFS made the news, Sun scuttled away from its "the default" quote. Only hours after his initial posting, Sun's Hamilton revised his blog so that the ZFS line read: "Jonathan noted that Apple is planning to use the ZFS file system from OpenSolaris in future versions of their OS [emphasis mine]." In the blog comments, he also added: "I hope this clears up some of the confusion and concern I may have caused."

So is ZFS in or out?
Cupertinologists are betting it's in, but not as the default file system. HFS+ still rules. Again, Harris on StorageMojo: "I'll stick to my prediction that Apple, as with HFS+, will put ZFS on OS X Server first before bringing it out later for the great unwashed." But Harris does see one possibility for those "unwashed" users: ZFS would be a great fit for flash disks, the nonmechanical drives making their way into laptops. The big reasons for a no-go on ZFS seem to focus on effort required versus time available, Apple's penchant for homegrown core technologies rather than slapping someone else's in place, and Time Machine's actual mechanics. On the latter, the best analysis remains Siracusa's from August 2006. With Apple, however, there's always a caveat. Maybe this is one of the Leopard pieces that Jobs wouldn't divulge last year for fear "... our friends in Redmond [will] start their photocopiers." And though it may be a coincidence, a revised Apple patent application was made public last month that could play to ZFS. This application specs out in-place file system conversion - it uses the example of converting Microsoft's FAT32 file system (the default in older editions of Windows) to Apple's HFS+ - so users wouldn't have to wipe and reinstall to switch. One interesting line in the application: "In general, any file-system used to organize and store files can be converted based on the location of the files(s) which is typically readily obtainable from the original file-system." HFS+ to ZFS, anyone?

What does Apple have to say about this? Nothing. But Jobs' keynote at WWDC starts at 10am (PDT) today (that's 6pm, UK time), and if ZFS is to be "the" Mac's file system, we'll hear about it then. Of course, if you believe in the Tao of FSJ (Fake Steve Jobs), this blog entry in The Secret Diary of Steve Jobs gives some good clues. Money quote, as FSJ would say: "As soon as I figure out what this thing is, I'll let you know."