Hello Chris,
The way the stacking cables works is it detects when you are using two of them and switches over to what is called 'ring' mode. It doesn't use both links at once, but if one of those links fails it will immediately use the second one, usually not even dropping one packet. STP isn't an issue on the stack ports because it doesn't really use it, the stacking is a proprietary protocol that handles all of this for you. So go ahead and plug up your second set of cables, you should see a log message saying you have moved from 'chain' to 'ring'.
As for LAGs, this is a common misconception with link aggregation. I understand the logic, you have 4 gig links, why can't you get 4 gigs of throughput? It has to do with how LAGs actually work.
When a packet arrives at the switch and needs to go out the LAG the switch runs a calculation on the source and destination IP (or source and destiantion MAC depending on your settings). From this calculation it comes up with a number, in your case from 1-4. That determines the link that it uses to get across that LAG. There is no way to make one conversation use more than one link, it just isn't how it works. So when you run a speed test from one IP to one other IP, the result of the switch's calculation will always be the same number, hence all the traffic for that stream goes down the same link in the LAG.
There isn't any spillover, so even if one link is much busier then the rest it doesn't move them over to one of the other links.
On some enterprise switches you can use a load balancing algorithm that uses source and destination port, which can result in multiple conversations between the same two clients to use multiple links, but on the Small Business switches we only have IP or MAC.
Basically any one conversation will always be limited to whatever the speed of 1 link in the LAG is.
It is possible to utilize the link more, but you would need several different computers talking to the server to really see that. QoS would not change this.
As for your speed, it is a bit low, the max for a gigabit link is about 118MB/s (lab max, your mileage may vary) if you are using normal sized frames. This is assuming two devices directly connected to the with no routing required. How are you testing the speed? I like to use a program called Tamosoft throughput tester for this, which is available free on the web.
Hope that helps a bit,
Christopher Ebert - Advanced Network Support Engineer
Cisco Small Business Support Center
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