ClickHouse Troubleshooting: Deep Dive into ClickHouse Wait Events


ClickHouse Wait Events are specific events or conditions that can cause the system to pause or wait during query execution. These events can significantly influence the performance of ClickHouse, as they impact query execution times and overall system responsiveness. Understanding and monitoring these wait events can help identify performance bottlenecks and optimize the system for better query throughput. 

Types of Wait Events in ClickHouse

Below is a tabular format explaining ClickHouse Wait Events and their influence on performance:

Wait EventDescriptionInfluence on Performance
Read from DiskWait for reading data from the disk.Can lead to slower query execution times if the disk I/O is slow or if data is not efficiently cached in memory.
Read from ZooKeeperWait for reading data from ZooKeeper (used for ReplicatedMergeTree).Can cause delays in replication and data consistency if ZooKeeper access is slow or unstable.
Read from NetworkWait for data to be received over the network.Can cause slow query execution if network bandwidth is limited or network latency is high.
MergeWait for merge operations to complete (used for MergeTree).Can lead to slower data merging, impacting the performance of data rollups and merges.
SortWait for sorting data during query execution.Can significantly slow down queries that involve sorting large amounts of data.
Wait for SpaceWait for available space in the MergeTree table for inserting data.Can cause delays in data ingestion if the MergeTree table is reaching its storage limits.
Wait for ReplicasWait for replicas to respond during replication (used for ReplicatedMergeTree).Can lead to replication delays and potential data inconsistency if replicas are slow or unavailable.
Wait for QuorumWait for the required number of replicas to achieve a quorum (used for ReplicatedMergeTree).Can cause write delays and impact data consistency in replication scenarios.
Write to ZooKeeperWait for writing data to ZooKeeper (used for ReplicatedMergeTree).Can introduce write delays and affect data consistency if ZooKeeper access is slow or unstable.
Wait for Table Structure LockWait for the table structure lock when altering table schema.Can cause delays in altering table structures and impact concurrent DDL operations.
Distributed SendWait for sending data in distributed query execution.Can slow down query execution when sending data to remote nodes in distributed queries.
Distributed Wait for MergeWait for data merging in distributed query execution.Can cause delays in distributed query execution if data merging is slow on remote nodes.
Distributed Wait for FetchWait for fetching data in distributed query execution.Can impact distributed query performance if data fetches are slow from remote nodes.


Monitoring and understanding these wait events can help pinpoint performance bottlenecks in ClickHouse and guide optimization efforts. For instance, addressing slow disk I/O, network congestion, or replication delays can lead to faster query execution and improved system responsiveness. By addressing these wait events, you can enhance the overall performance and efficiency of ClickHouse for real-time analytics and data processing.

To know more about Wait and Lock events in ClickHouse, do read the following articles:

About Shiv Iyer 227 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.