6/11/2023 0 Comments Bandwidth limitFigure 1 shows a scenario of congestion spreading caused by Slow Drain devices, which can lead to severe fabric-wide performance degradation.įigure 1: Congestion spreading Implementing per initiator bandwidth limitsĬustomers can now configure per initiator bandwidth limits at an Initiator Group (IG) or Host level. We have provided customers the ability to leverage Unisphere or Solutions Enabler QoS settings to keep faster array SLICs from overwhelming slower host HBAs. This application limits the amount of data that is sent to the end device (host) at a rate at which it can receive data. On PowerMax, this can be accomplished by applying a per initiator bandwidth limit. It does this through a mechanism where the customer can prevent an end device from becoming a Slow Drain by limiting the amount of data that will be sent to it. The goal of the Foxtail release is to reduce Slow Drain issues, as they are difficult to prevent and diagnose. Too much congestion can lead to congestion spreading, which can affect unrelated server and storage ports to experience performance degradation. Since the HBA is only capable of receiving data at 8 GB, there will be congestion at the host end. In short, Slow Drain is caused by an end device that is not capable of accepting data at the rate that it is being received.įor example, if an 8 GBs HBA sends a series of large block read requests to a 32Gbs PowerMax front-end Coldspell SLIC, the transition rate will be 32 GBs when the Array starts to send the data back to the host. This causes overall SAN degradation as central components, such as switches, encounter resources that are monopolized by traffic that is destined for the slow drain device, impacting all SAN traffic. Slow Drain is an FC SAN phenomenon where a single FC end point, due to an inability to accept data from the link at the speed at which it is being sent, causes switch/link buffers and credits to be consumed, resulting in data being “backed up” on the SAN. While there are many definitions and descriptions of Slow Drain, in this blog we define Slow Drain as: Customers have been burdened by this Slow Drain phenomenon for many years and this issue can lead to severe fabric wide performance degradation. From a PowerMax or VMAX AF perspective, this can help customers alleviate congestion spreading caused by Slow Drain devices, such as HBAs that have a lower link speed compared to the storage array SLIC. This new feature allows customers to leverage QoS settings on the PMAX/VMAXAF to reduce or eliminate Slow Drain issues due to servers with slow HBAs. This blog discusses a recently introduced functionality for PowerMax and All Flash arrays as part of the Foxtail release.
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