As processing demands increase, what looked yesterday like white hot technology has a sort of cool air to it, and physical limitations of an I/O spec can become painfully real. Hardware cannot be considered in isolation, and particularly if a distributed, multi vendor computing environment is to be supported, then thought should be given to what type of interface is and will be supported by a drive. Bandwidth is not the only consideration, issues such as the maximum distance between peripherals and the server, and the physical cable media required all come into play. The general issues of networking hardware will be looked at in a future edition of PDM, for the meantime we'll just take a quick look at some of the options for connecting tapes and disks to the workstation.
The current standard interface is SCSI which in its latest manifestations (WIDE and ULTRA WIDE) supports cable lengths of around 1.5 metres, and maximum bandwidth of 40 Megabytes per second (MB/s). Next in the pecking order comes Quantum's Low Voltage Differential SCSI which has been pushed to give 80 MB/s over a 25 metre cable. Fiber Channel is the ultimate specification today, offering 160 MB/s over 500 metre distances, and thus being well suited to a file serving rôle on a large departmental server.
Exponential growth
Fiber Channel was initially approved as a standard in 1993 and in 1994 Sun and HP formed the Fiber Channel Systems Initiative. The rational behind this alliance was the exponential growth in demand for storage capacity. By 1998, an IDC estimate shows some 500,000 hosts using Fiber Channel for a market of 2 billion dollars world wide. Drivers in the design of Fiber Channel were the aforementioned needs for capacity and greater physical distance, particularly in the context of networked multi-media, video on demand and Inter/Intranet. The ultimate potential of a Fiber Channel network is staggering. The scaleable architecture offers Gigabit bandwidth over 10 kilometers and uses a DB9 type 4 wire connector. Networks and peripherals can share the same physical medium. Ancor are very active in this field and have a Fiber Array product consisting of up to 112 drives of 9 GB capacity giving around 1TB per controller. A neat safety device is a battery backed up cache of 128 MB, so that if a controller goes down, the cache can be removed and the controller swapped out. This functionality if of considerable importance in transaction processing, where your bank is for instance crediting your account.
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