On Wed, May 28, 2014 at 3:08 PM, Matthew W. Ross <[email protected]> wrote: >> Latency is the real killer. > > How do you reduce latency?
Make the cable shorter. Ha ha, only serious. Network latency is how long it takes for a bit to go from point A to point B. The longer the path, the longer it takes. There are other factors. Signals propagate at different rates depending on the medium (e.g., CAT5 is very roughly half the speed of light). Processing (routing, switching, conversion, etc.) are typically slower than the wire, and can add delay, especially if the equipment is heavily loaded. My comment was directed more at WAN/Internet links. People see the ads for 42 bajigabits per second from cheap ISPs, and don't appreciate that there is more to speed than how wide the road is. > I decried to bypass the 2510G switch they > were aggregated through directly to our 5308xl. Guess what, the problem > all-but disappeared. Obviously, the 2510 can't handle the traffic. I wouldn't be so sure. On paper, the 2510 has a higher backplane bandwidth, and a higher forwarding rate. Comparing specs between vendors can be dicey, because of different test methodologies, but vendors are usually (*usually*) consistent within themselves. I'm more inclined to believe a configuration issue, physical problem (bad cable/port/etc.), or a local overload you bypassed. For example: If you've got six servers, each with a gigabit link, plugged into your 2510, and then have a single gigabit link connecting your 2510 to your 5308xl, the traffic from those servers could overwhelm the uplink. The buffer on the 2510 will fill and then start dropping frames. The servers go into retransmit and make the problem worse. Everything goes into exponential backoff until aggregate transmit rate drops below the uplink speed. Things start working again, so transmit rate increases. Lather rinse repeat. Another scenario would you've got something defined on the switch that forces the forwarding decision to go off the switch ASIC and onto the general-purpose CPU, which is orders of magnitude slower. Not saying these were your problem. They're just scenarios for the sake of discussion. Did you check port statistics on switches and servers? Check the logs on the switches? Are all the fault finders enabled on the switches? HP tech support should be able to help, too. > On a similar note, We used to be running HP 2910s as our iSCSI traffic > connection switches between our 3 VMware hosts and our EqualLogic, but we > were again having latency issues. Define "latency issues". Please note that "latency" does not mean "network trouble". :) > As recommended by our vendor, we upgraded > to Dell PowerConnect 6224s. On paper, these two switches are quite similar, > but the Dell switches handle the latency where the HPs were not. Uh.... "handle the latency"? :) Latency isn't something switches handle. It's something they cause, but usually not significantly. The specs for those switches do look pretty similar, so I too would expect them to handle similar loads. When you get into performance tuning of networks, configuration can matter a lot. Stuff that was wrong before but didn't cause a problem because it was small becomes a problem because now it's big. And different vendors each have different design assumptions, defaults, and quirks. As a result, you can get situations where vendor X works with vendor Y "out of the box", but fails with vendor Z. So you can replace all your X with Z, or you can adjust configuration. And then what do you do when you discover Z doesn't work with W? -- Ben
