Storage Networking Protocol Fundamentals (Vol 2)
| Early computer systems used various storage media such as punch cards, magnetic tapes, magnetic drums, and floppy magnetic disks for primary storage of data and programs. When IBM invented the hard disk drive in the early 1950s, a new era in storage began. The key benefits introduced include the following:
Some of these features were supported by other storage media, but the hard disk was the first medium to support all of these features. Consequently, the hard disk helped make many advances possible in computing. As evolutionary advances in hard disk technology came to market, they enabled further advances in computing. Intelligent Interface Models
One of the most important developments in storage technology was the advent of the intelligent interface model. Early storage devices bundled the physical interface, electrical interface, and storage protocol together. Each new storage technology introduced a new physical interface, electrical interface, and storage protocol. The intelligent interface model abstracts device-specific details from the storage protocol and decouples the storage protocol from the physical and electrical interfaces. This allows multiple storage technologies such as magnetic disk, optical disk, and magnetic tape to use a common storage protocol. It also allows the storage protocol to operate over different physical and electrical interfaces. Serial Transmission Techniques
Another key development in storage was the adoption of serial transmission techniques. Storage interfaces traditionally employ parallel electrical transmission techniques. Parallel electrical transmission has two significant limitations: bandwidth and distance. A common characteristic of electrical signals transmitted in parallel is electromagnetic interference among the wires in a sheath (called crosstalk). Crosstalk increases as the transmission rate increases. The ever-increasing capacities of modern disk drives necessitate ever-increasing transmission rates in their interfaces. Crosstalk makes it difficult to meet this challenge with parallel electrical transmission techniques. One countermeasure for crosstalk is to shorten the link distance, but distance constraints limit the usefulness of the interface. Cost control is also challenging. The economics of very long-distance cable infrastructure prohibit adoption of parallel transmission techniques. The serial nature of the global voice communications infrastructure provides evidence of this. Even for relatively short distances, such as data center requirements, parallel cabling costs can be adverse. Moreover, parallel cabling presents a logistical challenge. It can be difficult to fit through tight spaces or to bend around corners. As a result, the storage industry has embraced several serial transmission techniques in recent years. Serial electrical transmission techniques reduce the number of required conductors to one or two per direction. This does not eliminate crosstalk but significantly reduces it. Some of the adopted serial transmission techniques employ optical signaling, which completely eliminates crosstalk. Serial cabling is also less expensive and much easier to install and maintain. These benefits notwithstanding, some standards bodies have chosen to continue work on parallel electrical transmission techniques. The debate about the future of parallel technologies is still active, though serial technologies now clearly have the momentum. Modern Storage Networks
The adoption of the intelligent interface model and serial transmission techniques made it possible to leverage standard networking technologies for storage connectivity. The loop (also known as ring) topology was common in the early years of open systems storage networking but has since been broadly supplanted by switch-based solutions. Solutions based on Fibre Channel (FC) and Ethernet are most common today. |
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