etcd faq part 4 heartbeat and snapshotting
https://etcd.io/docs/v3.4/faq/
What does the etcd warning “failed to send out heartbeat on time” mean?
etcd uses a leader-based consensus protocol for consistent data replication and log execution. Cluster members elect a single leader, all other members become followers. The elected leader must periodically send heartbeats to its followers to maintain its leadership. Followers infer leader failure if no heartbeats are received within an election interval and trigger an election. If a leader doesn’t send its heartbeats in time but is still running, the election is spurious and likely caused by insufficient resources. To catch these soft failures, if the leader skips two heartbeat intervals, etcd will warn it failed to send a heartbeat on time.
Usually this issue is caused by a slow disk. Before the leader sends heartbeats attached with metadata, it may need to persist the metadata to disk. The disk could be experiencing contention among etcd and other applications, or the disk is too simply slow (e.g., a shared virtualized disk). To rule out a slow disk from causing this warning, monitor wal_fsync_duration_seconds (p99 duration should be less than 10ms) to confirm the disk is reasonably fast. If the disk is too slow, assigning a dedicated disk to etcd or using faster disk will typically solve the problem. To tell whether a disk is fast enough for etcd, a benchmarking tool such as fio can be used. Read here for an example.
The second most common cause is CPU starvation. If monitoring of the machine’s CPU usage shows heavy utilization, there may not be enough compute capacity for etcd. Moving etcd to dedicated machine, increasing process resource isolation with cgroups, or renicing the etcd server process into a higher priority can usually solve the problem.
A slow network can also cause this issue. If network metrics among the etcd machines shows long latencies or high drop rate, there may not be enough network capacity for etcd. Moving etcd members to a less congested network will typically solve the problem. However, if the etcd cluster is deployed across data centers, long latency between members is expected. For such deployments, tune the heartbeat-interval configuration to roughly match the round trip time between the machines, and the election-timeout configuration to be at least 5 * heartbeat-interval. See tuning documentation for detailed information.
If none of the above suggestions clear the warnings, please open an issue with detailed logging, monitoring, metrics and optionally workload information.
What does the etcd warning “snapshotting is taking more than x seconds to finish …” mean?
etcd sends a snapshot of its complete key-value store to refresh slow followers and for backups. Slow snapshot transfer times increase MTTR; if the cluster is ingesting data with high throughput, slow followers may livelock by needing a new snapshot before finishing receiving a snapshot. To catch slow snapshot performance, etcd warns when sending a snapshot takes more than thirty seconds and exceeds the expected transfer time for a 1Gbps connection.