Not all data is created equal, nor does all data require equal treatment. Some mainframe applications and their associated data require the absolute best performance, while others don’t. A FICON storage network has limited resources, so providing the highest performance to all applications may not be possible. This leads to decisions about Quality of Service (QoS) levels, monitoring, and enforcement. QoS, particularly in cascaded FICON architectures, is increasingly important to FICON customers.
There are different views on how to achieve FICON storage network QoS— ranging from implementing a strictly fabric-based QoS using Fibre Channel Class 4 Class of Service to implementations of QoS based on the Infiniband QoS concepts of Virtual Lanes (VLs) to a host-managed, true, end-to-end, QoS implementation.
This article examines QoS in a FICON-connected z/OS environment. It will review some basic QoS concepts and look at how storage network QoS is being addressed today. It also will review QoS-enabling ESCON features that are part of Workload Manager (WLM) and Intelligent Resource Director (IRD) such as dynamic channel path management and channel priority I/O queuing.
A follow-up article will explain how and why these features could be used in FICON environments to provide the best end-to-end QoS performance, availability, functionality, and manageability, and describe some best practices. Such host-to-device QoS based on techniques originally used in IBM’s WLM and IRD with ESCON appear to be the best way to achieve a true, end-to-end, QoS FICON solution.
Defining Quality and Service
Quality and service have different meanings in different contexts. A good, general definition of service is: the expected behavior or outcome from a system. So QoS is the degree to which the expected outcome is realized. Quantifying and measuring QoS also becomes a context-dependent task; it means different things to different people. For example, to the casual Internet user browsing a news site, QoS may simply mean the responsiveness of the Web server to his/her page accesses. On the other hand, to a systems administrator, QoS may mean the throughput and availability of the Web server, the network connection, stor- age subsystem, or some combination of these. To achieve a desired level of service, all the components on the end-toend path must be able to deliver that level of service.
Huseyin Simitci, in his book, Storage Network Performance Analysis (Wiley, 2003), defined some additional key concepts concerning QoS in storage network architectures:
• QoS architecture: The system must include the structures and interfaces to request, configure, and measure QoS. If the system’s peak performance is below the desired level, no amount of management can provide QoS.
• Admissions policy: This is a critical aspect of a QoS system. When a system accepts to serve (admit) a request, it must ensure certain resources are available to achieve the requested QoS level. If there aren’t enough resources, or if using the existing resources will hamper the QoS guarantees of previously admitted requests, the new arrivals should be rejected.
• Resource reservation: After a request is admitted to the system, sufficient system resources must be reserved to provide QoS to that request.