How prefetching works during WAL recovery in ClickHouse?


In ClickHouse, the Write-Ahead Log (WAL) is used to ensure the durability of data by logging all changes to the data before they are committed to disk. During WAL recovery, the WAL is replayed to restore the data to its consistent state in the event of a crash or other failure.

Prefetching and WAL performance

Prefetching is a technique used in ClickHouse to improve the performance of WAL recovery by reading data from disk ahead of time. The idea behind prefetching is to read data from disk into memory before it is actually needed, so that the data is available immediately when it is needed. In ClickHouse, prefetching is implemented using asynchronous I/O operations, which allow the WAL recovery process to continue reading data from disk even if the data has not yet been loaded into memory. The prefetched data is stored in a buffer, and the WAL recovery process can start processing the data as soon as it is available in the buffer. This allows the WAL recovery process to continue processing data even if the disk is slow, reducing the overall recovery time.

To ensure that the prefetching works efficiently, ClickHouse uses a multi-threaded approach, where multiple threads are used to prefetch data from disk. This allows the prefetching process to take advantage of multiple disk spindles and the full bandwidth of the disk subsystem, further reducing the recovery time.It’s important to note that the efficiency of prefetching during WAL recovery in ClickHouse depends on various factors, such as the size of the buffer used for prefetching, the number of threads used for prefetching, the performance of the disk subsystem, and the size of the WAL.

To optimize the prefetching process, it is important to choose an appropriate buffer size that is large enough to store the prefetched data, but not so large that it consumes too much memory. Additionally, it is important to choose an appropriate number of threads for prefetching that is optimized for the performance of the disk subsystem.In addition to prefetching, ClickHouse also uses other techniques to optimize the WAL recovery process, such as compression and parallelism. By combining these optimization techniques, ClickHouse is able to achieve fast and efficient WAL recovery, even for large data sets and high-throughput systems.


It’s also important to note that WAL recovery is a critical part of the database system and it’s essential to ensure that the WAL recovery process is reliable and efficient. To ensure the reliability of WAL recovery, it is important to regularly backup the WAL and test the recovery process to ensure that it works correctly.

To read more about Logs in ClickHouse, do read the below article:

About Shiv Iyer 222 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.