Device Mapper facility makes it possible to create pseudo block devices, that redirects IO to other block devices. While similar functionality exists in Solaris today (SVM) , it is not compatible with other popular operating systems (like Linux). Ability to access storage entities created under Linux will be beneficial for many users.
Generic device mapper facility in Solaris will create a foundation for providing other layers both for interoperability purposes (LVM2, etc) and other Solaris
native uses (think lofi being just a wrapper around device mapper for example).
The plan is to create Solaris native kernel device mapper driver, which will export user-land interface compatible with Linux device mapper, so that existing user-land tools could be easily ported to Solaris.
Having functioning device mapper facility is a prerequisite for creating a storage software stack compatible with Linux. Such stack will be attractive for people wanting to investigate transition from Linux to Solaris, or as a dual-boot solution.
Once device mapper is completed having LVM2-compatible facility should be mostly a mechanical work. Once LVM2 is available ext3 project will benefit from access to broader deployed Linux-based storage.