2.6 Mass Storage

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It is a given that an enterprise stores more data today than ever before. There are large databases, and numerous , sizeable flat files. Operating systems and executables (programs) are also larger than ever before.

Mass storage consists of more and more disk drives , collected and connected together to provide the space necessary for the large files and databases.

It started by filling a server s drive bay with SCSI disk drives. This was (and is) called embedded storage. Of course, storage requirements outgrew this arrangement, so manufacturers had a great idea: directly attached storage.

With directly attached storage, you merely installed a SCSI host bus adapter in the server and strung multiple SCSI drives together along the bus. But the next step was to put six or eight disk drives together into a single structure, called an enclosure. And the enclosures grew bigger, until they could hold 10 disk drives, then 20, then 30, and on and on. The trend continued by connecting multiple enclosures on the bus.

The disks in these disk farms were highly independent of each other, and together they were just a bunch of disks. This has given us one of the storage industry s best acronyms, JBOD ( Just a Bunch Of Disks ). SCSI JBODs remain a popular and effective method of attaching a lot of mass storage to a server.

Each of these disk drives could store massive amounts of data, massive being relative to the time. A big disk drive in the early 1990s might have been 500 MB in capacity. In the mid-1990s, we had passed the 1 GB, 2 GB, 4 GB, and 9 GB capacity points.

Currently, an HP JetStore 4000 disk enclosure holds six 9 GB or 18 GB drives, yielding up to 108 GB of storage. An SC60 disk enclosure has up to sixty 47 GB drives, providing about 2.8 TB.

2.6.1 Limitations of SCSI Mass Storage

Good as it sounds, the SCSI mass storage architecture has its limitations.

Limited speed. You might see 80 MBps on a good day.
Limited distance between devices. The SCSI cable limit is 25 meters .
Limited number of devices supported. The single-ended SCSI protocol is limited to 8 IDs or addresses per bus (7 devices and 1 controller), and wide differential SCSI is limited to 16 IDs per bus (15 devices and 1 controller). Configuring a 1 TB disk storage unit, a fully redundant disk array would require 30 SCSI IDs (two per bus).

These concerns about limited speed, distance, and number of devices caused the industry to start thinking about alternatives. The alternative is Fibre Channel.

2.6.2 Fibre Channel

Fibre Channel is a relatively new communications protocol designed to overcome the limitations of existing architectures. It is a generic data transport mechanism with the primary task of transporting data at the fastest rate possible using current technology. That rate is 1 Gbps or about 100 MBps. Fibre Channel is a scalable interface for achieving high-speed data transfer rates among heterogeneous systems and peripherals. System types include supercomputers, mainframes, workstations, and desktops (PCs).

Peripherals include mass storage devices such as disk arrays and tape libraries. Fibre Channel is a transport mechanism. It carries any number of existing protocols over a variety of physical media and existing cable options. But as it turns out, SCSI over fiber optic cable is the most popular choice.

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