The alluring promise of Linux on the mainframe is the ability to consolidate the tens, hundreds, or even thousands of discrete servers that have populated many machine rooms onto a single managed platform. While it is one thing to move the server function, the network, and the system management, a key piece is missing from this puzzle. What about all of the data on which these servers operated?
In many instances, this data may have been sitting on Small Computer Systems Interface (SCSI) devices in a Storage Area Network (SAN). However, up until now, it has not been possible for a Linux system running on the mainframe to get its hands on this data. Thus, the Total Cost of Ownership (TCO) equation, a prime driving force for Linux on the mainframe, is negatively affected by the need to acquire extra equipment and migrate data.
This situation has not gone unnoticed by IBM. In July 2002, they commenced an early support program that provided SCSI connectivity to its zSeries boxes. Indeed, at Linux World in 2002, IBM demonstrated a prototype of this support when it used its z900 mainframe to burn CDs on a SCSI-attached CD-writer. Practical? Hardly. Cool? Definitely!
This article describes the SCSI support provided to the zSeries architecture and provides a quick look at how it works as well as how well it works.
Software AG participates in many of IBM’s early support programs, as it needs to keep on the bleeding edge of technology so that its customers can move to the “latest and greatest” whenever it is generally available. Software AG was also one of the first Independent Software Vendors (ISVs) to announce support for Linux on the mainframe. So, IBM approached us to stress test its SCSI support. For this test, however, we would not be doing anything as esoteric as CD burning; just regular disk operations using our IBM Shark and EMC Symmetrix subsystems and tape operations using a 3590E.
What is IBM’s rationale for providing SCSI support? According to several IBM presentations, it is to enable Linux for zSeries to do SCSI-based I/O on volumes of arbitrary size and thus be able to access distributed storage existing in open SAN configurations. In addition, Linux for zSeries would have access to low-cost storage controllers and new storage devices not supported in classical zSeries configurations (i.e., optical libraries, CD, and DVD). The goal is to facilitate Linux and Unix server consolidation by hosting multiple Linux systems on a single zSeries server while protecting the investment organizations have made in existing SCSI storage.
A New CHPID
IBM has used its existing FIbre CONnections (FICON) technology as the basis of its SCSI regime. The FICON card runs new microcode that implements the Fibre Channel Protocol (FCP). FCP support for zSeries has been implemented by providing a new Channel Path Identifier (CHPID). Figure 1 shows an example of its definition within an IOCDS. This channel uses either the two-port fibre channel cards FICON or FICON Express. Currently, these cards operate at 1 Gbit per second, but 2Gbit per second support has been announced. The loading of different firmware into the card activates the FCP support. The QDIO protocol is used for communicating between processor, memory, and channel. This is the same protocol used by the OSA express card. This protocol uses continuously running channel programs to reduce I/O path lengths and the number of interrupts.