Networking on z/OS
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Queued Direct I/O (QDIO)

Networking on z/OS

QDIO is a highly efficient data transfer architecture, which dramatically improves data transfer speed and efficiency for TCP/IP traffic.

QDIO mode is referred to as OSD because the CHPID type coded in the IOCDS is OSD.

Figure 1 illustrates the much shorter I/O process when in QDIO mode compared with non-QDIO mode (the same I/O path as the OSA-2 features). Consequently, I/O interruptions and I/O path-lengths are minimized.

The advantages of using QDIO mode are:

  • A 20% improvement in performance versus non-QDIO mode
  • The reduction of system assist processor (SAP) utilization
  • Improved response time
  • Server cycle reduction

How is this all accomplished? Instead of attaching an OSA-Express card using a channel, and hence utilizing a channel or I/O program (IOP) combined with CCW operations, the OSA-Express card attaches using an STI bus. STI stands for Self-Timed Interface. The OSA-Express card is still connected within the I/O cage, but the STI bus is directly connected to the memory bus of the CPC.

What is used instead of a CCW operation to signify that I/O needs to be completed? With an OSA-Express card running in QDIO mode, I/O operations are effected using a signal adapter instruction, or SIGA. The SIGA is still processed by the SAP, similar to the way a CCW is processed by the SAP. However, the SIGA effectively passes a pointer to the data because the data already occupies internal storage.

This bus itself has a data speed of up to 2.7 GBps (gigabytes per second), which is plenty of bandwidth to handle a 10 Gbps Ethernet LAN speed.

Figure 1. Non-QDIO versus QDIO data pathsNon-QDIO versus QDIO data paths

QDIO incorporates a number of features:

  • LPAR-to-LPAR communication

    Access to an OSA-Express port can be shared among the system images that are running in the LPARs to which the channel path is defined to be shared. Also, access to a port can be shared concurrently among TCP/IP stacks in the same LPAR, in different LPARs, or in different logical channel subsystems. When port sharing, an OSA-Express port operating in QDIO mode has the ability to send and receive IP traffic between LPARs without sending the IP packets out to the LAN and then back to the destination LPAR.

  • DMA (direct memory access)

    DMA allows data to move directly from the OSA-Express microprocessor to the host memory. This bypasses three layers of processing that are required when using ESCON and OSA-2 features, dramatically improving throughput.

  • Priority queuing

    Priority queuing (for z/OS environments) sorts outgoing IP message traffic according to the priority assigned in the IP header (using the Type Of Service field). This priority is used to reflect the business priorities assigned to the application, user ID, time of day, and other characteristics.

  • Enhanced IP network availability

    Enhanced IP network availability (IPA) is a service of the QDIO architecture. When TCP/IP is started in QDIO mode, it downloads all the home IP addresses in the stack and stores them in the OSA-Express feature. The OSA-Express feature port then responds to ARP requests for its own IP address, as well as for other IP addresses active in the TCP/IP stack (in particular with virtual IP addresses (VIPA).

  • VLAN support

    IEEE standard 802.1Q describes the operation of virtual bridged LANs, known as VLANs. A VLAN is defined to be a subset of the active topology of a LAN. The OSA-Express features provide for the setting of multiple unique VLAN IDs per QDIO data device. They also provide for both tagged and untagged frames to flow from an OSA-Express port.

    Full VLAN support is offered for all OSA-Express Ethernet features available on mainframe servers. z/OS Communications Server versions 1.5 and later support VLAN identifications (VLAN IDs).

    Reminder: A VLAN frame looks almost the same as an Ethernet frame. The difference is that a VLAN frame has a extra field containing a number that identifies the VLAN. This number is called a VLAN tag.
  • ARP Takeover

    The Address Resolution Protocol (ARP) Takeover provides the capability of switching OSA-Express port operations from one OSA-Express to another OSA-Express running in the same mode.

    When TCP/IP is started in QDIO mode, it downloads all the home IP addresses in the stack and stores them in each OSA-Express feature to which it has a connection. This is a service of QDIO architecture and occurs automatically only for OSD channels.

    If an OSA-Express feature fails while there is a backup OSA-Express available on the same network ID, TCP/IP informs the backup OSA of which IP addresses (real and VIPA) to take over, and the network connection is maintained. The takeover is effected by something called a gratuitous ARP. A gratuitous ARP is an unsolicited ARP response. All hosts on the LAN segment that receive this gratuitous ARP will update their ARP cache with the new MAC address for the backup OSA.

Copyright IBM Corporation 1990, 2010