Merge pull request #5899 from heyitsanthony/qos-tuning

Documentation: tuning advice for peer prioritization

Documentation/tuning.md

@@ -71,4 +71,23 @@ $ etcd --snapshot-count=5000
 $ ETCD_SNAPSHOT_COUNT=5000 etcd
 ```
 
+## Network
+
+If the etcd leader serves a large number of concurrent client requests, it may delay processing follower peer requests due to network congestion. This manifests as send buffer error messages on the follower nodes:
+
+```
+dropped MsgProp to 247ae21ff9436b2d since streamMsg's sending buffer is full
+dropped MsgAppResp to 247ae21ff9436b2d since streamMsg's sending buffer is full
+```
+
+These errors may be resolved by prioritizing etcd's peer traffic over its client traffic. On Linux, peer traffic can be prioritized by using the traffic control mechanism:
+
+```
+tc qdisc add dev eth0 root handle 1: prio bands 3
+tc filter add dev eth0 parent 1: protocol ip prio 1 u32 match ip sport 2380 0xffff flowid 1:1
+tc filter add dev eth0 parent 1: protocol ip prio 1 u32 match ip dport 2380 0xffff flowid 1:1
+tc filter add dev eth0 parent 1: protocol ip prio 2 u32 match ip sport 2739 0xffff flowid 1:1
+tc filter add dev eth0 parent 1: protocol ip prio 2 u32 match ip dport 2739 0xffff flowid 1:1
+```
+
 [ping]: https://en.wikipedia.org/wiki/Ping_(networking_utility)