Quick answer: Yes, there is DB2 LUW on the mainframe under zLinux

One of the harder Google and Bing searches is to find out whether DB2 for Linux, UNIX, and Windows (DB2 LUW) is supported on a Linux system running on the mainframe (System z). Often only references to DB2 for z/OS or DB2 on Linux are found. Anyway, to answer a question that I got, DB2 for Linux is supported on the so-called zLinux and it might come to a surprise that you can choose between DB2 for z/OS (with a deep system integration) or DB2 for the distributed platform.