Introduction
PageIOLatch waits are a type of wait event that occurs when a thread is waiting for a page to be read from disk into memory. In ClickHouse, these waits are implemented as part of the storage engine, which manages the reading and writing of data to disk.
When a query is executed in ClickHouse, the storage engine first checks if the required data is already in memory. If the data is not in memory, the engine must read it from disk. During this process, if the required page is already being read by another thread or process, the current thread will have to wait for the page to be loaded into memory before it can proceed with the query execution.
The PageIOLatch wait event is used to indicate that a thread is waiting for a page to be loaded into memory. When a thread encounters a PageIOLatch wait, it is temporarily blocked until the required page is loaded into memory. Once the page is loaded, the thread is unblocked and can proceed with the query execution.
ClickHouse uses a number of techniques to minimize PageIOLatch waits and improve query performance. For example, ClickHouse’s data compression and caching features can help reduce the amount of data that needs to be read from disk, thereby reducing the frequency of PageIOLatch waits. Additionally, ClickHouse’s multi-threaded architecture allows multiple threads to read data from disk in parallel, which can help reduce the overall time spent waiting for data to be loaded into memory.
In summary, PageIOLatch waits in ClickHouse are implemented as part of the storage engine’s data reading and writing processes, and are used to indicate when a thread is waiting for a page to be loaded into memory. ClickHouse uses a number of techniques to minimize PageIOLatch waits and improve query performance, including data compression, caching, and multi-threaded data reading.
Monitoring ClickHouse PageIOLatch waits
ClickHouse provides a system table called system.events which can be used to monitor PageIOLatch waits and other system events. You can use a SQL query to filter and aggregate the events table to retrieve the necessary information.
Here’s an example SQL query that you can use to monitor PageIOLatch waits in ClickHouse:
SELECT event_date, event_time, query_id, event_type, event_subtype, thread_id, event_info FROM system.events WHERE event_type = 'Wait' AND event_subtype = 'PageIOLatch' ORDER BY event_time DESC LIMIT 100;
This query retrieves the most recent 100 PageIOLatch wait events from the system.events table. The event_date and event_time columns indicate when the wait event occurred, the query_id column identifies the query that triggered the wait event, and the thread_id column identifies the thread that was waiting for the page to be loaded. The event_info column provides additional information about the wait event, such as the name of the database or table being accessed.
You can modify this query to filter and aggregate the events table as needed to monitor PageIOLatch waits over a longer time period or for specific queries or tables. For example, you can group the results by query_id or thread_id to identify the most frequent or longest-running wait events.
Conclusion
Note that the system.events table may not be enabled by default in ClickHouse, depending on the configuration of your installation. You may need to modify the configuration file or use the CREATE TABLE statement to enable the events table before you can use this query.
To read more about Locks & Waits in ClickHouse, do consider reading the below articles
