you should be able to get at least 8MBps (64Mbit) from memory cache over the network (to null device), and with the ftp over tcp/ip protocoll you should be able to get transfer rates of data up to 11Mbps (88Mbps), which is around the max for 100Mbps ethernet, since there also is overhead
the DU meter is good for troubleshooting when there's a problem, but it's not the most accurate way of measuring speeds. If you really want to measure something correctly, I would download something through ftp from one pc to the other, to the null device and from cache (make sure there's plenty of ram in the pc which runs the ftp server, and transfer the same file a couple of times after another, to make sure it reads from cache, which can be verified when the hd led stays off on the server ). The client's hd led also shouldn't light up when downloading to the null device
the ftp server software should report the average transfer speed achieved. You also shouldn't download small files, since the smaller the file, the more inaccurate the result. Try a single file around 50 - 150 MB (as large as possible while still being loaded from diskcache on the server).
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