I’m sure we all agree Information Technology (IT) and acronyms are strongly tied together since the beginning. Considering the amount of Three Letter Acronyms (TLAs) we can build is limited and now exhausted, it comes with no surprise that FLAs are the new trend. You already understood that FLA means Four Letter Acronym, right? But maybe you don’t know that the ancient Romans loved four letter acronyms and created some famous ones: S.P.Q.R. and I.N.R.I.. As a technology innovator, Cisco is also a big contributor to new acronyms and I’m pleased to share with you the latest one I heard: DIRL. Pronounce it the way you like.
Please welcome DIRL
DIRL stands for Dynamic Ingress Rate Limiting. It represents a nice and powerful new capability coming with the recently posted NX-OS 8.5(1) release for MDS 9000 Fibre Channel switches. DIRL adds to the long list of features that fall in the bucket of SAN congestion avoidance and slow drain mitigation. Over the years, a number of solutions have been proposed and implemented to counteract those negative occurrences on top of Fibre Channel networks. No single solution is perfect, otherwise there would be no need for a second one. In reality every solution is best to tackle a specific situation and offer a better compromise, but maybe suboptimal in other situations. Having options to chose from is a good thing.
DIRL represents the newest and shining arrow in the quiver of the professional MDS 9000 storage network administrator. It complements existing technologies like Virtual Output Queues (VOQs), Congestion Drop, No credit drop, slow device congestion isolation (quarantine) and recovery, portguard and shutdown of guilty devices. Most of the existing mitigation mechanisms are quite severe and because of that they are not widely implemented. DIRL is a great new addition to the list of possible mitigation techniques because it makes sure only the bad device is impacted without removing it from the network. The rest of devices sharing the same network are not impacted in any way and will enjoy a happy life. With DIRL, data rate is measured and incremental, such that the level of ingress rate limiting is matched to the device continuing to cause congestion. Getting guidance from experts on what mitigation technique to use remains a best practice of course, but DIRL seems best for long lasting slow drain and overutilization conditions, localizing impact to a single end device.