Disk storage has changed rapidly over the past few years with the delivery of new functions and improved performance. DB2 has made many changes to keep pace and make use of the disk improvements. In fact, there is a new "red paper" that outlines these changes titled Disk storage access with DB2 for z/OS.
A red paper is similar to a redbook, but it is shorter (in length) and focuses on a more limited subject matter. They are kind of in between a white paper and a redbook. But they are still free to download - and they are easier to digest quickly than a redbook.
If you don't know what a redbook is, check out my earlier blog posting on the subject: Can You Write a Redbook?