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@(#)motivation.ms 6.2 (Berkeley) 4/16/91
$FreeBSD$
Motivation
These benchmarks were performed for several reasons. Foremost was our desire to obtain guideline to aid in choosing one the most expensive components of any VAX UNIX configuration, the disk storage system. The range of choices in this area has increased dramatically in the last year. DEC has become, with the introduction of the UDA50/RA81 system, cost competitive in the area of disk storage for the first time. Emulex's entry into the VAX 11/780 SBI controller field, the SC780, represented an important choice for us to examine, given our previous success with their VAX 11/750 SC750 controller and their UNIBUS controllers. The Fujitsu 2351A Winchester disk drive represents the lowest cost-per-byte disk storage known to us. In addition, Fujitsu's reputation for reliability was appealing. The many attractive aspects of these components justified a more careful examination of their performance aspects under UNIX.
In addition to the direct motivation of developing an effective choice of storage systems, we hoped to gain more insight into VAX UNIX file system and I/O performance in general. What generic characteristics of I/O subsystems are most important? How important is the location of the controller on the SBI/CMI versus the UNIBUS? Is extensive buffering in the controller essential or even important? How much can be gained by putting more of the storage system management and optimization function in the controller as DEC does with the UDA50?
We also wanted to resolve particular speculation about the value of storage system optimization by a controller in a UNIX environment. Is the access optimization as effective as that already provided by the existing 4.2BSD UNIX device handlers for traditional disks? VMS disk handlers do no seek optimization. This gives the UDA50 controller an advantage over other controllers under VMS which is not likely to be as important to UNIX. Are there penalties associated with greater intelligence in the controller?
A third and last reason for evaluating this equipment is comparable to the proverbial mountain climbers answer when asked why he climbs a particular mountain, ``It was there.'' In our case the equipment was there. We were lucky enough to assemble all the desired disks and controllers and get them installed on a temporarily idle VAX 11/780. This got us started collecting data. Although many of the tests were later rerun on a variety of other systems, this initial test bed was essential for working out the testing bugs and getting our feet wet. .bp