do-we-need-qos-in-data-center.html

---
date: 2015-02-27T08:57:00.000+01:00
tags:
- data center
- load balancing
- QoS
title: Do We Need QoS in the Data Center?
url: /2015/02/do-we-need-qos-in-data-center/
---

<p>Whenever I get asked about QoS in the data center, my stock reply is “<em>bandwidth is cheaper than QoS-induced complexity</em>.” This is definitely true in most cases, and ideally the <a href="/2014/06/mice-elephants-and-virtual-switches/">elephant problems</a> should be <a href="/2015/01/load-balancing-elephant-storage-flows/">solved higher up in the application stack</a>, not with network-layer kludges, but are there situations where you actually need data center QoS?<!--more--></p>
<p><a href="http://en.wikipedia.org/wiki/Explicit_Congestion_Notification">Congestion detection and TCP ECN marking</a> might be a good use case and can be done with minimal interface configuration – all it takes is a few configuration lines on Arista EOS and Cisco Nexus OS. <a href="https://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01">Data Center TCP</a> uses ECN markings to detect congestion and reduce transmission rate before packets get dropped (packets drops could result in <a href="/2014/03/per-packet-load-balancing-interferes/">not-so-insignificant performance degradation because they kick the NICs out of TCP offload mode</a>).</p>
<p>There might be cases where you need QoS to reduce latency, but I don’t think VoIP qualifies. At 10Gbps speeds, you need 1 MB of packets sitting in the output queue to generate an additional millisecond of latency.</p>
<p>Finally, if you’re forced to implement queuing to reduce the impact of elephant flows (for example), insist on behaving like a service provider and keeping the network configuration as clean as possible – police ingress traffic (if needed) and queue packets based on <a href="/2013/09/overlay-networks-and-qos-fud/">DSCP or 802.1p markings</a>. Application-aware processing (hopefully resulting in DSCP marking) <a href="/2014/07/vmware-vswitch-and-8021p-cos-value/">belongs to hypervisors</a> or the end-hosts, not to the ToR switch.</p>
<p>Anything else? Share your thoughts in the comments.</p>