Previously, I'd seen figures on the order of 30 MB/s single drive to single drive over a number of tests, and had accepted this as a general constant of sorts. Based on the above report, I decided to test some more -- I broke up one of my RAID arrays into individual drives, and shucked files around.
Now I see some variability. Some of that seems to come from the tools used -- xxcopy seems to give more variability than xcopy for example. (So I'll stick with xcopy for now.) I've also see results that I just can't explain at this time (particularly bad performance). I'll work on this some more (I think this might be related to SATA drivers and other configuration parameters).
But the purpose of my note is to report that I also see high network transfer rates for single drives that are consistent with drive performance, without using jumbo frames.
I've seen 50-60 MB/s transfer rates, single drive to single drive for transferring an 8.4 GB file (with RAM = 2 GB) around the beginning of the drive. This is roughly consistent with transfer rates within drives on the same machine without networking, so indicates that there is not a great deal to be gained in my case with jumbo frames or other network tweaks. These drives bench around 65 MB/s STR at the beginning, and 35 MB/s at the end, so this would be the expected range of file transfer performance for files that aren't significantly fragmented and the drives are free to do sequential transfers without much seeking. Previously, in at least some tests, I'd used older / more crowded drives, so perhaps gotten the 30 MB/s figures because of that. 30 MB/s is still a valid figure for the end of my newer drives, but I now see that even single drives can do much better in some cases.
Jumbo frames become important when some part of the system / networking / even CPU perhaps can't keep up with a large volume of packets. So I don't dispute that it might be a very important factor for some cases, just that it isn't a single magic bullet, and isn't necessary in all cases.
Thanks for the push to re-test. There's more to learn, and perhaps I'll be able to use jumbo frames eventually and see benefits from them in some cases.