Linux Connectivity options when running under VM

The following are connectivity options available when running Linux as a guest of VM. Listed with each option are advantages or disadvantages associated with each option.

  • IUCV - this is a point to point connection. This may be a problem if you have lots of Linux guests and there are some 64-bit limitations.
  • vCTC - also point to point. IUCV, in some cases, is cheaper but not in all cases.
  • Guest LAN - is currently the strategic choice for talking between guests. The choice between HiperSockets Guest LAN and GbE Guest LAN is dictated more by function than by performance. There is a slight advantage to HiperSockets Guest LAN in terms of shorter pathlength in some cases, and the larger available MTU may help.
  • HiperSockets - this is the best choice for connections between LPARs
  • A virtual Router to connect to a real network (OSA GbE) is often used. The Linux stack has more function although the VM stack has lower pathlength in many cases. New with z/VM 4.4.0 is the Virtual Switch which is a good alternative to using a virtual machine Router.
  • Each Linux guest can have direct connections with the lowest overhead. However, this uses up a lot of storage below the 2GB line (approximately 2000 pages per Linux Guest). It can also make 'wiring' a bit more challenging. That's why many use virtual routers or will use the new Virtual Switch.

A Guest LAN is a virtual network formed by connecting z/VM guests on the same host system using virtual network adapters rather than dedicated hardware network adapters. In this network, one virtual network adapter must be defined in each z/VM guest machine. Each adapter then connects to a z/VM Guest LAN segment. z/VM can emulate two kinds of network adapters:

  1. HiperSockets adapters, which are optimized for exchanging large unicast, broadcast, and multicast packets between known IP addresses.
  2. QDIO adapters, which provide more general support, including broadcast, multicast, and routing functions.

A z/VM Guest LAN can connect to a physical LAN through a TCP/IP router. With a Guest LAN, data flows through the router to move between the OSA Express device to the Guest LAN.

A z/VM Virtual Switch can be created as a special type of Guest LAN. In addition to providing a network of virtual adapters, the switch can be connected directly to an OSA Express QDIO adapter. This capability allows you to gain connectivity to external LAN segments without requiring a router. For more information refer to z/VM Virtual Networking Hints and Tips

Deployment of a virtual switch reduces the CPU utilization cost and latency associated with providing external connectivity through a router virtual machine. The switching logic resides in the z/VM Control Program (CP) which owns the OSA-Express connection and performs all data transfers between Guest LAN nodes and the OSA-Express. This eliminates the overhead associated with a router

The following picture illustrates a Guest LAN and a virtual switch. The difference is that a router is not needed when using a virtual switch.

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