What is a Slow Draining Device?
A slow draining device is a device connected to a network which delays the return of Receiver-Ready (R_RDY) primitives in order to slow down the rate at which the frames arrive.
This happens more often on the read channel because it’s more common to have older hosts communicating with newer arrays. But it is also possible to have an array be a slow drain due to a newer host, write cache issues or front-end processor saturation.
One slow draining device can negatively impact wide swaths of the fabric. It is much more impactful with ISLs involved where the ISLs begin blocking
- Slow Drainer sends a Read request to Array 1
- Data flows with no problems from the Array 1 to the Core Switches
- Data flows with no problems from the Core Switches to the Edge switches
- Slow Drainer can’t keep up with data it requested, which causes flow control mechanism to slow down the traffic.
- Data stays in the buffers on Edge Switches and they can’t accept more data coming from Core Switches
- Data stays in the buffers on Core Switches and they can’t accept more data coming from the Array 1
- Victim sends a request to the Array 2
- Array 2 sends data to Core Switches but they can’t send it further because of the the ongoing B2B Credit Starvation.