Free tutors on the SNIA Shared Storage Model and The functional approach

Free tutors on the SNIA Shared Storage Model and The functional approach

The fact that there is a lack of any unified terminology for the description of storage architectures has already become apparent at several points in previous chapters. There are thus numerous components in a storage network which, although they do the same thing, are called by different names. Conversely, there are many systems with the same name, but fundamentally different functions. A notable example is the term' data mover' relating to server-free back-up (Section 7.8.1) in storage networks. When this term is used it is always necessary to check whether the component in question is one that functions in the sense of the 3rd-party SCSI Copy Command for, for example, a software component of back-up software on a special server, which implements the server-free back-up without 3rd-party SCSI. This example shows that the type of product being offered by a manufacturer and the functions that the customer can ultimately expect from this product are often unclear. This makes it difficult for customers to compare the products of individual manufacturers and find out the differences between the alternatives on offer. There is no unified model for this with clearly defined descriptive terminology. For this reason, in 2001 the Technical Council of the Storage Networking Industry Association (SNIA) introduced the so-called Shared Storage Model in order to unify the terminology and descriptive models used by the storage network industry. Ultimately, the SNIA wants to use the SNIA Shared Storage Model to establish a reference model, which will have the same importance for storage architectures as the seven-tier OSI model has for computer networks. In this chapter, we would first like to introduce the disk-based Shared Storage Model (Section 10.1) and then show, based upon examples (Section 10.2), how the model can be used for the description of typical disk storage architectures. In Section 10.3 we introduce the extension of the SNIA model to the description of tape functions. We then discuss examples of tape-based back-up architectures (Section 10.4). Whilst describing the SNIA Shared Storage Model we often refer to text positions in this book where the subject in question is discussed in detail, which means that this chapter also serves as a summary of the entire book.

THE MODEL

In this book we have spoken in detail about the advantages of the storage-centric architecture in relation to the server-centric architecture. The SNIA sees its main task as being to communicate this paradigm shift and to provide a forum for manufacturers and developers so that they can work together to meet the challenges and solve the problems in this field. In the long run, an additional reason for the development of the Shared Storage Model by SNIA was the creation of a common basis for communication between the manufacturers who use the SNIA as a platform for the exchange of ideas with other manufacturers. Storage-centric IT architectures are called shared storage environments by the SNIA. We will use both terms in the following. First of all, we will describe the functional approach of the SNIA model (Section 10.1.1) and the SNIA conventions for graphical representation (Section 10.1.2). We will then consider the model (Section 10.1.3), its components (Section 10.1.4) and the layers' file/record layer' and 'block layer' in detail (Section 10.1.5 to Section 10.1.8). Then we will introduce the definitions and representation of concepts from the SNIA

model, such as access paths (Section 10.1.9), caching (Section 10.1.10), access control (Section 10.1.11), clustering (Section 10.1.12), data (Section 10.1.13) and resource and data sharing (Section 10.1.14). Finally, we will take a look at the service subsystem (Section 10.1.15).

The functional approach

The SNIA Shared Storage Model first of all describes functions that have to be provided in a storage-centric IT architecture. This includes, for example, the block layer or the file/record layer. The SNIA model describes both the tasks of the individual functions and also their interaction. Furthermore, it introduces components such as server ('host computer') and storage networks ('interconnection network'). Due to the separation of functions and components, the SNIA Shared Storage Model is suitable for the description of various architectures, specific products and concrete

installations. The fundamental structures, such as the functions and services of a shared storage environment, are highlighted. In this manner, functional responsibilities can be assigned to individual components and the relationships between control and data flows in the storage network worked out. At the same time, the preconditions for interoperability between individual components and the type of interoperability can be identified. In addition to providing a clear terminology for the elementary concepts, the model should be simple to use and, at the same time, extensive enough to cover a large number of possible storage network configurations. The model itself describes, on the basis of examples, possible practicable storage architectures and their advantages and disadvantages. We will discuss these in Section 10.2  without evaluating them or showing any preference for specific architectures. Within the model definition, however, only a few selected examples will be discussed in order to highlight how the model can be applied for the description of storage-centred environments and further used.