Not sure about running on Linux, but there are COBOL and CICS systems that run on small servers. In fact there are a whole range of mainframe-like machines (e.g

Not sure about running on Linux, but there are COBOL and CICS systems that run on small servers. In fact there are a whole range of mainframe-like machines (e.g. AS/400) from IBM, to support different size businesses. The amount of data in some applications IS immense. If you are managing a multi-million-account credit card portfolio on a single mainframe, which nicely supports all your realtime apps (including a 100-request per second authorizations system) and your 6-hour batch cycle, there is no reason at all to port that to smaller machines. The mainframe is marvelously stable and nowadays the whole machine rarely gets hung up (I haven't seen or heard of this happening to my company or its customers since the early 90's). Sure, you could move those apps to a bunch of smaller boxes. Then you open yourself up to server failure, admin costs of maintaining multiple boxes, etc. Mainframes wrote the book on SaaS long before 'net cloud computing came along. IBM and others have operated computing as a utility for years and years. What manager who gets his IT done on rented computer time is going to suggest taking the apps in-house on a bunch of servers? None that I know of.