Why congestion bottlenecks are dangerous, too
seb_ 060000QVK2 Visits (7254)
Almost a year ago I wrote an arti
Well...stop! Didn't you talk about congestion bottlenecks?
Yes! Today I want to explain how a congestion bottleneck could cause the exact same symptoms on the devices like a latency bottleneck - and exactly the same performance degradations. This is how it happens. In the middle you see a SAN director with 2 portcards and 2 core cards. While the devices are connected to the portcards, the core cards provide the backend connections between them. They are internally connected via the backplane. So for example host 1's way over there to the storage array A would traverse the portcard, then one of the both core cards and leaves the other portcard until it reaches storage array A. It could even be that two devices connected to the same portcard have to go over the core cards, because so called local switching is only done within an ASIC and a portcard could have more than one depending on the number of ports.
Now please meet host 2. Host 2 is a wonderful modern server. One of the work horses of the datacenter. It's fully packed with virtual machines, but its many cores and memory, as well as its state-of-the-art HBA, provide enough horsepower to cope with the workload. This baby is more than capable to do the work and it's in no way a slow drain device. It's zoned and mapped to the storage arrays A, B, C and D and it uses them heavily, mostly for read operations. The green tiny bars are read requests and as you see in the next picture it sends them to all of them, all of the time.
But the more and longer the link
The crux is: you will hardly find a congestion bottleneck that just flows with high link utilization and no negative effects. The probability is much higher for the following scenario:
Frames pile up inside the storage arrays and other end devices impaired by the slow drain behavior. If their RAS package is good, they will yell about credit starvation and probably even drop frames within their FC adaptors. In extreme situations these frame drops could happen in the director, too. At least you would see then something that would point you to a performance problem. Because otherwise - if you would have substantial delay in the traffic but all the frames get finally transferred to the next internal or external hop within 500ms ASIC hold time - you would only see the congestion bottleneck. And without bottleneckmon you wouldn't see anything at all then. The switch would look clean. Nothing in porterrshow or porstatsshow. Both show only external port counters anyway. As a SAN administrator you would not suspect anything in the director to cause this.
And still it would be there. A big performance problem caused by a device communicating with too much other devices. Not a slow drain device but still causing a slow drain in the SAN.
So how to solve it?
It's basically what