ClickHouse/doc/reference/en/introduction/use_case.rst

Usage in Yandex.Metrica and other Yandex services
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ClickHouse is used for multiple purposes in Yandex.Metrica. Its main task is to build reports in online mode using non-aggregated data. It uses a cluster of 374 servers, which store over 20.3 trillion rows in the database. The volume of compressed data, without counting duplication and replication, is about 2 PB. The volume of uncompressed data (in TSV format) would be approximately 17 PB.

ClickHouse is also used for:
 * Storing WebVisor data.
 * Processing intermediate data.
 * Building global reports with Analytics.
 * Running queries for debugging the Metrica engine.
 * Analyzing logs from the API and the user interface.

ClickHouse has at least a dozen installations in other Yandex services: in search verticals, Market, Direct, business analytics, mobile development, AdFox, personal services, and others.