Well, in case you haven't read any of the myriad stories about it, it appears that Apple has decided not to use ZFS on Mac OS X. Gruber has sources that say it was primarily licensing concerns, which is consistent with what people have implied to me, both recently, and around WWDC (although at that time I think there was probably still hope of resolving the issues).
Now, some people jump may comment that it couldn't be licensing issues, since ZFS is opensource (under the CDDL), and that Apple already uses CDDL software (DTrace). That may be true, but often in deals that involve large companies there is more to it than that. Apple may have wanted guarantees of indemnification in the NetApp lawsuit. Maybe it wanted guarantees that certain modifications it wanted to make would be accepted upstream, or even to get Sun to make certain changes. It also might have wanted additional distribution rights that were not granted under the CDDL. It is typical for companies to negotiate custom agreements in such cases (and for some money to change hands), so the idea that licensing issues are why it fell through is entirely reasonable, even though it is an opensource product. Obviously Sun's steady decline in the market place, and the uncertainty caused by the Oracle acquisition may have greatly complicated any such negotiations.
Why not do a new filesystem?
Apple has a lot of talented filesystem engineers. They are certainly capable of doing something comparable to ZFS, at least for their target market. The problem with developing a new modern filesystem is that it generally takes longer than a single OS release cycle. Most companies are really bad at having large teams focused on projects that will not ship in the next version of the project they are working on.
This is a particularly acute problem at Apple, which traditionally has done things with very few engineers. I don't want to get into exact numbers, but I recall having a discussion with the head of a university FS team who was discussing the FS he was working on. He was pitching it to a group of Apple engineers. It was some interesting work, but there were some unsolved problems. When he was asked about them he commented that they didn't have enough people to deal with them, but he had some ideas and it shouldn't be an issue for a company with a real FS team. It turned out his research team had about the same number of people working on their FS as Apple had working on HFS, HFS+, UFS, NFS, WebDAV, FAT, and NTFS combined. I think people don't appreciate how productive Apple is on a per-engineer basis. The downside of that is that sometimes it is hard to find the resources to do something large and time consuming, particularly when it is not something that most users will notice in a direct sense. That is especially true if senior management is not excited about the idea.
Because of that, I was fairly convinced ZFS was a credible future primary FS for Apple. Not because it was an optimal design for them (it isn't), but because it was a lot less work than doing a new design from scratch. The fact its fundamental architecture is 20 years newer than HFS meant it would still be better than HFS+ in almost all respects even if it was not designed for Apple's exact needs. Clearly I was wrong, since Apple has stopped the ZFS project.
Well, a couple of things have happened. The first is that Mac OS X has gotten more mature. They no longer need to port all of those FSes, they already have them working, and in most cases they work fairly well. That frees up some engineers. Apple has also greatly expanded the number of people working on their kernel since it is amortized over many different products (Mac OS X, iPhone, AppleTV, etc).
Suddenly the notion of doing a new filesystem seems doable, so long as it is a real priority and the FS team doesn't get pulled to keep adding features or doing major work to legacy FSes. That is still a lot of work when Apple had ZFS approaching production quality on OS X.
Apple can do better than ZFS
Sun calls ZFS "The Last Word in Filesystems", but that is hyperbole. ZFS is one of the first widely deployed copy on write FSes. That certainly makes it a tremendous improvement over existing FSes, but pioneers are the ones with arrows in their back. By looking at ZFS's development it is certainly possible to identify mistakes that they made, and ways to do things better if one were to start from scratch. From where I sit, there are 3 obvious ways doing a new FS will be better for Apple than ZFS:
There have been new fundamental research since ZFS was designed that simplifies many of the issues involved with it. In particular the "B-trees, Shadowing, and Clones" (PDF). That paper is the basis for the design of BtrFS, which has a very similar feature set to ZFS, but internally is entirely different. LWN has an article about BtrFS that explains the significance in some detail (it is written Valerie Aurora, who worked on ZFS at Sun).
ZFS was designed for the storage interfaces available a decade ago. Spinning disks are going to be with us for a long time, especially for bulk storage in data centers and on backup devices. The future is all about solid state. Flash SSDs have significantly different performance characteristics than spinning media, and there may be FS design decisions one could make that would benefit from that. Now, any FS Apple designs will have to work acceptably on traditional drives, but if they are designing for the future then flash is what to target.
ZFS has had some optimization work for flash, but it is all in terms of using flash as part of a storage hierarchy. That makes complete sense, since ZFS's primary deployment targets are high-end systems and data center storage. Those systems have multiple drives, so the idea of separate flash drives for a ZIL and L2ARC are completely reasonable. Most consumers have one drive in their system, and maybe an external drive for bulk data, data exchange, and backup.
That brings up the last point. ZFS is designed for big systems. It works on small systems, but most of the tradeoffs favor very large computers, with lots of drives. This shows up in a number of ways. The first is that ZFS is not currently capable of adding single drives to an existing vdev or migrating vdevs between various types (mirror, raidz, raidz2). This is a major feature for smaller users who might want to add a single drive, but is a non-issue for data center users who tend to add large number of drives all at once, since they will add whole vdevs. Another issue is that ZFS assumes you have a lot of ram. NEC has been doing a port of OpenSolaris to ARM, and they determined they could not get ZFS to use less than 8 megabytes of ram without making incompatible format changes (Compacted ZFS). With those changes they could squeeze it into a more reasonable 2 megabytes. On a desktop that doesn't seem like a big deal, but on an iPhone 3G or a Time Capsule 8MB of wired memory is an enormous issue.
The only major downside is that if Apple is just starting on a next generation FS now it could be a long time before we get our hands on it.
But now we are going to have another incompatible next generation filesystem
Wolf brought this point up during some of the ZFS talk on twitter yesterday. My general opinion is that it doesn't matter. People use drives for two largely unrelated tasks. One is running their computers. This is fixed storage. The other is for data exchange. In the old days people used floppies for their sneakernet media, which made the situation much simpler to understand. In recent years the market realities have caused people to move to using SD cards, thumbdrives, and hard drives as the exchange medium of sneakernet.
The important point is that understand is that while the physical devices may be the same, the use model is different, just as the using a floppy disk and an internal hard drive were different. Nobody would balk at the notion that floppies should use different FSes than internal drives. Likewise, most people shouldn't care that their external drives are formatted differently than their internal drives.
There are complicated features you want for your boot drives and system disks. Ideally you could have them on your interchange disks, but there are other features that are more important, particularly interoperability, and simplicity. ZFS didn't bring either of those. There might have been a few people who were psyched to be able to use ZFS to share disks between a Mac and a Solaris or FreeBSD box, but honestly those people are few and far between. Whether Apple used ZFS or something else it is just as interoperable with Linux and Windows (which is to say, not at all). So that fact that Apple looks to be doing a new FS does not impact interoperability in any real sense.
The other feature you really want for an interchange FS is simplicity. There are a lot of devices out there that use an FS to communicate with a computer. The simplest example is a digital camera via its media cards, but there are many others. Something like ZFS is way too complex for those devices, and honestly most of the features of ZFS like multiple drive support and snapshots are useless since the devices don't have the physical interconnects or user interfaces to expose those features. There is certainly an argument to be made that we could use something a bit better than FAT32 or exFAT as that format, but ZFS was not the right solution for that.
In other words, for that disk you want to use as an external drive to drag between computers you don't want something like ZFS, you want something that is simple enough that a firmware engineer can write a read-only implementation from the specs in less than a week. For the disk embedded in your computer (operationally or literally) you want something like ZFS, but it doesn't matter if it is interoperable with anything else because you won't be moving it between systems.
This is basically how Windows works. Microsoft generally uses NTFS for internal drives, but FAT for external drives. Ultimately somebody should design a filesystem explicitly for use as an interchange format and license it for free, then everyone can deal with their internal FSes and do what makes the most sense for their OSes and markets.