I doubt they'll make ZFS GPL. ZFS is a really big reason to use Solaris, and unless they plan on killing Solaris, Linux's paws need to remain off of ZFS. On the other side of things, Oracle has a LOT of Linux installations, and the features of ZFS that btrfs is bringing on board are unbelievably useful, especially in a database environment.
With btrfs, periodic snapshots can be done without touching or "notifying" the database, in O(1) time. I know that I've setup a PostgreSQL box on ZFS to do snapshots every 5 minutes, just because it's so "free." Most database software has some sort of built-in mechanism to do snapshot backups, but they're generally quite expensive, especially when compared to snapshots in a COW filesystem like ZFS or btrfs.
What you're saying is "They won't make ZFS GPL because they want to keep people on Solaris, but they'll develop btrfs because they want to use it on Linux".
When the two FSes are so similar, what's the point of developing them both? They either want the features on Linux, so they should GPL ZFS, or they don't, and they should discontinue btrfs. Either way, btrfs loses, but either way, we win (btrfs won't stop being developed, so we'll get a great FS one way or the other).
Yes, that's exactly what I'm saying. If they GPL ZFS, they end up risking a major reason people choose Solaris. ZFS is very much a "brand" among systems administrators. It took quite a while for Sun to convince people to put their precious business critical data on ZFS, and it'll be the same story for btrfs.
Both of these projects are costing them so little, it's much easier to justify keeping them going than it is to take on the unknown business risk of making ZFS directly merge-able into the Linux kernel. Sun clearly chose the CDDL to keep this from happening, and post-merger, you've probably got the same people in charge of the Solaris and ZFS projects, so strategies aren't going to change overnight.
tl;dr: Oracle has low, predictable expenses for development of both btrfs and ZFS, and there's no reason to rock the boat for <1m/year in expenses.
With btrfs, periodic snapshots can be done without touching or "notifying" the database, in O(1) time. I know that I've setup a PostgreSQL box on ZFS to do snapshots every 5 minutes, just because it's so "free." Most database software has some sort of built-in mechanism to do snapshot backups, but they're generally quite expensive, especially when compared to snapshots in a COW filesystem like ZFS or btrfs.