Shadow Files Instead of Snapshots

Hercules does not use snapshots like other virtualization programs, but it does implement shadow files, which can be put to similar use. To use a shadow file, you need to change the lines with the hard disk configuration slightly (Listing 2, line 29ff.):

0A01 3390 dasd/sys1.dasd ro sf=shadow/sys1_*.dasd
0A02 3390 dasd/sys2.dasd ro sf=shadow/sys2_*.dasd

After starting, Hercules automatically writes the shadow/sys?_1.dasd files. The sf+ * command at the Hercules console creates a new set of shadow files, up to a maximum of eight files. sf- * merge or sf- * nomerge accepts or discards the changes to the current shadow step. Accepting means that sf- merge writes the changes from, for example, sys1_2.dasd to sys1_1.dasd. To write changes to the original files, you must specify sf- * force (or omit the read-only option ro).

The Hercules suspend and resume commands save and restore the complete system state; however, this mechanism does not work reliably – zLinux will not work after you use it.

z/OS and MVS

From a Linux perspective, the mainframe is just one architecture out of many, so it makes sense to look at the operating systems that normally run on mainframes. On the current crop of systems, the operating system is z/OS 1.x. (Version 1.9 was released in September 2007.)

Unfortunately, z/OS is not free. Although there is an "ADCD" package for developers and a z/OS demo package, you at least need to lease an IBM machine to use it legally. This just leaves the granddaddy of today's z/OS, MVS (version 3.8), as an alternative for experimenting at home.

MVS (Multiple Virtual Storage) was developed from earlier IBM operating systems; its direct predecessor is the OS/360 MVT (Multitasking with a Variable number of Tasks) variant. MVS then became the operating system for the next generation of hardware, the System/370 computer, which supported virtual memory as of 1972.

Interestingly, the older S/360-67 models did support virtual memory, but at the time, batch processing was regarded as far more important and hardware support for memory virtualization was thus omitted from the S/370 series.

MVS was released in 1974 and led directly via MVS/XA, MVS/ESA, and OS/390 to z/OS. Now MVS is freely available because operating systems at the time were distributed openly.

A "turnkey" MVS system is available for Hercules: the MVS Turnkey System [7][8]. The only additional software you need is a 3270 terminal emulator; Linux users can install the x3270 package that comes with their distributions.

If you think you can set up a mainframe system quickly and start using it, you will be disappointed. Although you can boot and down the system using the "Cookbook" [8], it will not help you much if you do not have any previous mainframe experience. If you are seriously interested in immersing yourself in this material and working your way through the available documentation – including the current z/OS documentation, which is freely available from IBM – you will enjoy the experience of your own mainframe.

Conclusions

Hercules offers a stable emulator for mainframe architecture. Running on a powerful midrange system, the performance should be sufficient to replace a zLinux test LPAR. If you are thinking of porting your software to zLinux, you can use Hercules or the Community Development System for Linux (LCDS), a service provided by IBM [9].