128 lines
5.6 KiB
Plaintext
128 lines
5.6 KiB
Plaintext
.\" Copyright (c) 1983 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)conclusions.ms 6.2 (Berkeley) 4/16/91
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.\"
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.ds RH Conclusions
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.NH
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Conclusions
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.PP
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Peak available throughput is only one criterion
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in most storage system purchasing decisions.
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Most of the VAX UNIX systems we are familiar with
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are not I/O bandwidth constrained.
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Nevertheless, an adequate disk bandwidth is necessary for
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good performance and especially to preserve snappy
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response time.
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All of the disk systems we tested provide more than
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adequate bandwidth for typical VAX UNIX system application.
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Perhaps in some I/O-intensive applications such as
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image processing, more consideration should be given
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to the peak throughput available.
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In most situations, we feel that other factors are more
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important in making a storage choice between the systems we
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tested.
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Cost, reliability, availability, and support are some of these
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factors.
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The maturity of the technology purchased must also be weighed
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against the future value and expandability of newer technologies.
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.PP
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Two important conclusions about storage systems in general
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can be drawn from these tests.
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The first is that buffering can be effective in smoothing
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the the effects of lower bus speeds and bus contention.
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Even though the UDA50 is located on the relatively slow
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UNIBUS, its performance is similar to controllers located on
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the faster processor busses.
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However, the SC780 with only one sector of buffering shows that
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little buffering is needed if the underlying bus is fast enough.
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.PP
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Placing more intelligence in the controller seems to hinder UNIX system
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performance more than it helps.
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Our profiling tests have indicated that UNIX spends about
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the same percentage of time in the SC780 driver and the UDA50 driver
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(about 10-14%).
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Normally UNIX uses a disk sort algorithm that separates reads and
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writes into two seek order queues.
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The read queue has priority over the write queue,
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since reads cause processes to block,
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while writes can be done asynchronously.
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This is particularly useful when generating large files,
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as it allows the disk allocator to read
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new disk maps and begin doing new allocations
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while the blocks allocated out of the previous map are written to disk.
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Because the UDA50 handles all block ordering,
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and because it keeps all requests in a single queue,
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there is no way to force the longer seek needed to get the next disk map.
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This disfunction causes all the writes to be done before the disk map read,
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which idles the disk until a new set of blocks can be allocated.
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.PP
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The additional functionality of the UDA50 controller that allows it
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to transfer simultaneously from two drives at once tends to make
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the two drive transfer tests run much more effectively.
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Tuning for the single drive case works more effectively in the two
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drive case than when controllers that cannot handle simultaneous
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transfers are used.
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.ds RH Acknowledgements
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.nr H2 1
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.sp 1
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.SH
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\s+2Acknowledgements\s0
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.PP
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We thank Paul Massigilia and Bill Grace
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of Digital Equipment Corp for helping us run our
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disk tests on their UDA50/RA81.
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We also thank Rich Notari and Paul Ritkowski
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of Emulex for making their machines available
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to us to run our tests of the SC780/Eagles.
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Dan McKinster, then of Systems Industries,
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arranged to make their equipment available for the tests.
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We appreciate the time provided by Bob Gross, Joe Wolf, and
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Sam Leffler on their machines to refine our benchmarks.
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Finally we thank our sponsors,
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the National Science Foundation under grant MCS80-05144,
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and the Defense Advance Research Projects Agency (DoD) under
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Arpa Order No. 4031 monitored by Naval Electronic System Command under
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Contract No. N00039-82-C-0235.
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.ds RH References
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.nr H2 1
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.sp 1
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.SH
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\s+2References\s0
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.LP
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.IP [McKusick83] 20
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M. McKusick, W. Joy, S. Leffler, R. Fabry,
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``A Fast File System for UNIX'',
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\fIACM Transactions on Computer Systems 2\fP, 3.
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pp 181-197, August 1984.
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.ds RH Appendix A
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.bp
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