Tuning

The default settings in etcd should work well for installations on a local network where the average network latency is low. However, when using etcd across multiple data centers or over networks with high latency, the heartbeat interval and election timeout settings may need tuning.

The network isn’t the only source of latency. Each request and response may be impacted by slow disks on both the leader and follower. Each of these timeouts represents the total time from request to successful response from the other machine.

The underlying distributed consensus protocol relies on two separate time parameters to ensure that nodes can handoff leadership if one stalls or goes offline. The first parameter is called the Heartbeat Interval. This is the frequency with which the leader will notify followers that it is still the leader. For best practices, the parameter should be set around round-trip time between members. By default, etcd uses a heartbeat interval.

The second parameter is the Election Timeout. This timeout is how long a follower node will go without hearing a heartbeat before attempting to become leader itself. By default, etcd uses a 1000ms election timeout.

Adjusting these values is a trade off. The value of heartbeat interval is recommended to be around the maximum of average round-trip time (RTT) between members, normally around 0.5-1.5x the round-trip time. If heartbeat interval is too low, etcd will send unnecessary messages that increase the usage of CPU and network resources. On the other side, a too high heartbeat interval leads to high election timeout. Higher election timeout takes longer time to detect a leader failure. The easiest way to measure round-trip time (RTT) is to use .

The election timeout should be set based on the heartbeat interval and average round-trip time between members. Election timeouts must be at least 10 times the round-trip time so it can account for variance in the network. For example, if the round-trip time between members is 10ms then the election timeout should be at least 100ms.

The heartbeat interval and election timeout value should be the same for all members in one cluster. Setting different values for etcd members may disrupt cluster stability.

The default values can be overridden on the command line:

The values are specified in milliseconds.

etcd appends all key changes to a log file. This log grows forever and is a complete linear history of every change made to the keys. A complete history works well for lightly used clusters but clusters that are heavily used would carry around a large log.

To avoid having a huge log etcd makes periodic snapshots. These snapshots provide a way for etcd to compact the log by saving the current state of the system and removing old logs.

Creating snapshots with the V2 backend can be expensive, so snapshots are only created after a given number of changes to etcd. By default, snapshots will be made after every 10,000 changes. If etcd’s memory usage and disk usage are too high, try lowering the snapshot threshold by setting the following on the command line:

# Command line arguments:
$ etcd --snapshot-count=5000
$ ETCD_SNAPSHOT_COUNT=5000 etcd

On Linux, etcd’s disk priority can be configured with :

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:

As etcd is very sensitive to latency, performance can further be optimized on Linux systems by setting the CPU governor to performance or conservative mode.

On Linux, the CPU governor can be configured to performance mode:

echo performance | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor