An Investigation into Transport Protocols and Data Transport ...

A.1. Data Storage 266
the ICP cards and about 400Mbit/sec for the 3Ware cards. The trend of
the drop can be partially explained by file caching and also by the extra
overheads introduced by the writing of the parity information required for
RAID-5. The RAID-0 write distributions (Figure A.7) are typically much
flatter as parity information is not created.
The read speeds remain essentially constant for all file sizes, only decreasing
to lower speeds with file sizes smaller than 200MB. This can be
explained through the overheads of file seek and buffer management. It was
observed that the read 3Ware cards were able to achieve about 50% greater
read performance than that of the ICP cards at optimal vm.max-readahead
settings.
For all configurations, it was observed that an increase in the vm.max-readahead
value also increased disk read performance. As vm.max-readahead continues
to be set to higher and higher values the relative improvement in disk performance
diminishes. No significant improvements in disk performance were
achieved for vm.max-readahead values greater than 1200 and hence larger
values are not included. There appears to be no conclusive increase in the
write performance with a change in vm.max-readahead.
Figure A.1 and Figure A.2 show results for the ICP SATA RAID controller
configured as RAID-5 with PCI interface jumper settings of 33MHz and
66MHz respectively. The read speed clearly improves as vm.max-readaheadis
increased. The write speeds remained largely unaffected, although there was
a larger standard deviation of values at small file sizes for a vm.max-readahead
of 31. In both cases, there is no discernible increase in performance until the
vm.max-readahead was set above 127. The use of a 66Mhz PCI bus clearly
leads to an increase in performance in both read and write speeds. However,
it should be noted that a tuned 33Mhz ICP card is able to get the