An Investigation into Transport Protocols and Data Transport ...

A.1. Data Storage 265
The virtual memory setting in the Linux kernel vm.max-readahead was
varied to determine if any improvement in read and write speeds could be
achieved. The variable vm.min-readahead which controls the floor of the
vm.max-readahead settings was also investigated but was found to have negligible
effect and the results are therefore are not presented.
The read and write speed tests were performed for a distribution of file
sizes ranging from 100MB to 2GB with increments of 100MB in order to give
a realistic range of useable data file sizes.
Each measurement was repeated five times from which the arithmetic
mean value was calculated. This mean was used as the central value from
which to determine the standard deviation for the five measurements to determine
the standard error on the mean [Boa83].
A.1.3
Results
Figures A.1 to A.5 show a series of graphs for read and write speeds versus
file sizes for different settings of the Linux variable vm.max-readahead. Due
to hardware restrictions, only RAID-5 configurations with an array of 4 disks
were investigated.
Figure A.6 summarises the RAID-5 results achieved by the various controllers
with vm.max-readahead settings of 1200 and also includes 8-disk
arrays. Figure A.7 is the equivalent summary graph for RAID-0. The results
for 2GB files are also displayed in Table A.4 .
All graphs shown for read and write speeds show similar shapes: write
speeds typically begin at a relatively high value for small file sizes then drop
quite suddenly at around 400MB for all RAID-5 configurations. The graphs
flatten off to a relatively low value - approximately 500 to 600Mbit/sec for