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Network Concepts
The physical network in System z consists of devices known as Open Systems Adapters (OSAs). Several varieties are available, such as the OSA-Express4S and OSA-Express5S. These are capable of handling up to 640 TCP/IP stacks simultaneously, including HiperSockets for inter-LPAR communication. An IBM System zEC12 provides up to 96 OSA-Express5S ports for external network communications. The Open Systems Adapter supports both copper and fiber Ethernet connections at speeds of up to 10 Gb.
As might be expected, the z/VM feature to access the Internet Protocol network is TCP/IP for z/VM. OSA-Express devices can be virtualized through a virtual switch (VSWITCH) device to many Linux guests. It is available using special z/VM machines known as VSWITCH controllers. Each Linux guest connects using a virtual device controlled by the qeth module to a virtual switch system in a z/VM LPAR.
An important benefit of the VSWITCH system is that it can be set up with redundant OSA devices that provide a failover network system on z/VM
HiperSockets provide high-speed interconnectivity among guests running on a System z. This technology does not require any special physical device configurations or cabling. The guests simply communicate with one anot her internally via the in-memory capabilities of the PR/SM hypervisor. HiperSockets, however, are not intended to be used for sophisticated networking and should not be used for external traffic.
Both OSA-Express and HiperSockets use the Queue Direct I/O (QDIO) mechanism to transfer data. This mechanism improves the response time using system memory queues to manage the data queue and transfer between z/VM and the network device
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Network Facilities
On the Mainframe, quite a number of different network devices are available for use. Many of these come from a historical background, and should not be used for new implementations. They commonly stay, however, to continue the support of previous installations on newer hardware. Linux on System z can operate using all common network interfaces but for new installations, there are recommended methods for operation depending on the use case.
The following are some technologies that you will find in the System z wo rld that are not used or even seen on x86 systems. This section clarifies some new facilities that you are going to find when you are migrating from x86 to System z. We provide some brief information that you can use to start your network planning. In each subsection, you can find a reference for more detailed information.
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The Open Systems Adapter (OSA)
The Open Systems Adapter (OSA) is a hardware network controller. It is installed in a Mainframe I/O cage and provides connectivity to clients on local area networks (LANs) or wide area networks (WANs). It ca n be directly attached on Linux but will typically be attached to virtual switches (read more in the "Virtual switch" section below). You can find more technical information about OSA cards on IBM zEnterprise EC12 Technical Guide, SG24-8049.
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OSA with Link Agregation
You can aggregate multiple physical OSA cards in to a single logical link, which is called a link aggregation group (LAG). This configuration increases the bandwidth and provides nondisruptive failover. How to co nfigure it is well described on Advanced Networking Concepts Applied Using Linux on IBM System z, SG24-7995.
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HiperSockets
HiperSockets is a microcode implementation that emulates a Logical Link Control Layer of an OSA interface. HiperSockets provides near zero latency at memory speed communications between servers running in different LPARs. When connecting a Linux guest to an IBM z/OS system on the Mainframe, the HiperSockets network in Layer 3 mode is the method to use. HiperSockets must be configured in the I/O configuration of the Mainframe. HiperSockets do not provide external connections. If an external connection is required, either a HiperSockets bridge must be implemented by using a VSWITCH, or a Linux guest must be set up as a router.
HiperSockets provide a very fast connection between LPARs. They provide an easy way to connect many Linux servers to a z/OS system in the same Mainframe. This direct connection without involving real hardware is an important factor to simplify setups with many Linux systems that must be connected to z/OS. Some benefits are explained in Set up Linux on IBM System z for Production, SG24-8137.
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Virtual Switch
A virtual switch (VSWITCH) is a software program that enables one virtual host to communicate with another virtual host within a computer system. Virtual switches typically emulate functions of a physical Ethernet switch. In Linux on System z, a VSWITCH provides direct attachment of z/VM guests to the local physical network segment. The VSWITCH allows IP network architects and network administrators to treat z/VM guests as a server in the network.
The switched network inside a z/VM Operating System commonly is implemented with a VSWITCH. When running the VSWITCH as Layer 2, it behaves similar to a real switch just between virtual machines.
The actual speed of a connection with a VSWITCH depends on a number of different variables. The type of traffic is as important as the real underlying hardware and the maximum transmission unit (MTU), which is the maximum size (in bytes) of one packet of data that can be transferred in a network. Common to all of those solutions is that the VSWITCH is faster than a real switch connected to the Mainframe would be.
VSWITCHes do not need a connection to an OSA card to operate. They can also provide purely virtual networks. This also simplifies the setup of private interconnects between guest systems. When creating private interconnects in an SSI with LGR enabled, the use of dedicated VLANs with external interface is recommended. This is necessary to accomplish the private connection between guests that run on different nodes in the SSI.
Implementing VLANs also helps if different guests run in different security zones of a network. It is easy to configure network interfaces to Linux guests that provide only selected VLANs to the guest. These can be configured either as tagged VLANs or as single untagged VLAN on an interface.
The VSWITCH infrastructure provides two basic configuration options. One configures user-based access, the other configures port-based access. From the possibilities, both are equivalent, just the configurations differs.
You can read more about VSWITCH benefits on Set up Linux on IBM System z for Production, SG24-8137, and technical information about Advanced Networking Concepts Applied Using Linux on IBM System z, SG24-7995.