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Posted by Bob Willard on March 5, 2008, 7:17 am
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Stephanie wrote:
> We just upgraded the office network, pulling new cat-5e cable over the
> suspended ceiling and into the walls. All connections use quality hardware.
> The gigabit switch is a LinkSys SR2024:
>
> <http://tinyurl.com/39s8xr>
>
> The computers are 1-year-old Dell desktops running a mix of Win XP and 2K.
> These have Intel brand gigabit NICs installed.
>
> There is nothing between computers on the LAN except the SR2024.
>
> When copying a test file between computers, we're seeing 10-12
> megabytes/second. This isn't anywhere near what I was expecting. I realize
> that 1 gigabit speeds are only theoretical and reserved for only those
> optimized systems with the fastest busses and hard drives, but I think my
> numbers are a tad low.
>
> What speeds can we expect? What is slowing down this new network?
>
> Thanks.
>
First, make sure you are using a single large file, rather than a folder
full of small files, to minimize the effect of per-file overhead in the
filesystem. Also, make sure the file is contiguous, although a newly-
created file on a HD with lots of empty space almost surely is. And, make
sure the file is large enough that you can measure transfer time with a
watch (and then hand-calculate the STR, or Sustained Transfer Rate); some
apps are kinda sloppy about reporting STR.
Next, do your testing with only one OS, either XP or W2K; I've seen great
differences between mixed and pure OS results. {If achieving maximum
STR is really important, you should plan on replacing all W2K boxes with
XP boxes, and adding RAM. Did I mention Vista? Not.}
Next, note that there are four cases, and potentially four different STRs:
A pushing a file to B, A pulling a file from B, B pushing to A, and B pulling
a file from A.
Next, repeat the measurements several times, and use the highest STR as
the basis for comparison when you try something different.
Next, eliminate the switch by connecting a pair of your fastest XP PCs with a
crossover cable in place of the switch. Repeat the STR measurements to see
the "insertion loss" caused by the switch. I have no clue about that
particular switch, but it has to buffer at least a part of every incoming
packet to decide how to handle that packet, and that delays packet delivery,
even without congestion effects.
Finally, experiment with tuning the network drivers. Most importantly, pick
a size for jumbo frames and use the same size for your entire LAN. Increasing
the number of Xmt and Rcv transmit descriptors might help, but they chew up
RAM on the PCs. I suspect that you want both Xmt and Rcv flow control turned
on, but you might experiment with that.
Good luck. And please report back what you find and what you achieve.
--
Cheers, Bob
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