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Apple Seeds ZFS Read/Write Developer Preview 1.1 for Leopard
In the release notes, Apple confirms that the original release of Mac OS X Leopard (10.5) will only offer Read-Only ZFS. As a result, no ZFS pools or filesystems can be modified or created under 10.5.0. This developer's preview enables full read/write capability, including the creation/destruction of ZFS pools and filesystems.
ZFS is described as "a fundamentally new approach to data management. We've blown away 20 years of obsolete assumptions, eliminated complexity at the source, and created a storage system that's actually a pleasure to use."
The initial version of Mac OS X 10.5 Leopard is rumored to be released on October 26th, 2007. It's unclear when Leopard will incorporate full Read/Write ZFS support, but it seems clear that Apple is working on adding this functionality.