ClickHouse/docs/en/engines/table_engines/log_family/log_family.md

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31	Introduction

Log Engine Family

These engines were developed for scenarios when you need to quickly write many small tables (up to about 1 million rows) and read them later as a whole.

Engines of the family:

• StripeLog
• Log
• TinyLog

Common Properties

Engines:

• Store data on a disk.

• Append data to the end of file when writing.

• Support locks for concurrent data access.

  During INSERT queries, the table is locked, and other queries for reading and writing data both wait for the table to unlock. If there are no data writing queries, any number of data reading queries can be performed concurrently.

• Do not support mutation operations.

• Do not support indexes.

  This means that SELECT queries for ranges of data are not efficient.

• Do not write data atomically.

  You can get a table with corrupted data if something breaks the write operation, for example, abnormal server shutdown.

Differences

The TinyLog engine is the simplest in the family and provides the poorest functionality and lowest efficiency. The TinyLog engine doesn’t support parallel data reading by several threads. It reads data slower than other engines in the family that support parallel reading and it uses almost as many descriptors as the Log engine because it stores each column in a separate file. Use it in simple low-load scenarios.

The Log and StripeLog engines support parallel data reading. When reading data, ClickHouse uses multiple threads. Each thread processes a separate data block. The Log engine uses a separate file for each column of the table. StripeLog stores all the data in one file. As a result, the StripeLog engine uses fewer descriptors in the operating system, but the Log engine provides higher efficiency when reading data.

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