The Network File System (NFS) file sharing configuration that is described in 4.6.3, “Sharing zPDT DASD across PCs” on page 82 does not deliver response times that you often experience if all the systems that are accessing the disk were in the same PC. If the performance effect of our sample configuration is not acceptable, this appendix describes a more robust alternative.

We built our base sysplex with two Linux PCs, as shown in Figure 4-4 on page 80. This configuration is probably the simplest configuration possible and we use it for descriptive purposes. However, the performance of this configuration might not suit many users. An alternative base sysplex configuration is shown in Figure C-1. This configuration involves a separate “device” for network-attached storage (NAS).

We tested two NAS devices. One was a more sophisticated (and more expensive) unit with four drives (2 TB each) and four network interfaces (of which we used a single interface). The other NAS device was a much lower-priced unit with one 2 TB drive and one network interface. In both cases, our LAN connections were through 1 Gb switches or routers. All our emulated 3390 volumes were placed on the NAS device. All of the normal Linux files (including the zPDT programs, devmaps, and so forth) were retained on the separate PC systems.

There was a noticeable difference in performance between the more expensive and the less expensive NAS devices. The more expensive unit supported NFSv4. The less expensive unit supported only the older NFS operation.

We connected three base sysplex z/OS systems by using the more expensive NAS device. Performance was fairly good, although not as good as with locally resident PC disks. With the less expensive NAS device, we connected two base sysplex z/OS systems and performance was acceptable for many purposes. Performing an initial program load (IPL) shows the most noticeable slowdown, especially with the less expensive NAS device. When z/OS was ready, the systems were usable for our particular tasks.

These configurations involve an extra expense (the NAS device, with disk drives) and the extra complexity of integrating it into your configuration.¹ Many different NAS devices are available from multiple vendors. You must determine which, if any, meet your performance requirements.

Some common sense rules, such as not loading multiple z/OS systems within seconds of each other, can be helpful. The disk performance of zPDT is largely dependent on the effectiveness of the local Linux disk cache in each Linux machine. Operations that “drag” large numbers of modules from a remotely mounted disk, such as during IPL or the first TSO logon, can be considerably slower than later operations that benefit from having modules and data in the local cache.

The Linux and zPDT setup for using NAS storage is almost the same as for our base sysplex implementation as described in 4.6.3, “Sharing zPDT DASD across PCs” on page 82.

As their name implies, NAS devices are network-attached. If your network is the Internet, you must consider security and performance implications. Most vendor products offer VPN operation for better security, although a VPN might be slightly more difficult to configure. The local networks we used provided response times in the 0.15 millisecond range for simple ping operations.