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System Tables
Introduction
System tables provide information about:
- Server states, processes, and environment.
- Server’s internal processes.
System tables:
- Located in the
systemdatabase. - Available only for reading data.
- Can’t be dropped or altered, but can be detached.
Most of system tables store their data in RAM. A ClickHouse server creates such system tables at the start.
Unlike other system tables, the system log tables metric_log, query_log, query_thread_log, trace_log, part_log, crash_log and text_log are served by MergeTree table engine and store their data in a filesystem by default. If you remove a table from a filesystem, the ClickHouse server creates the empty one again at the time of the next data writing. If system table schema changed in a new release, then ClickHouse renames the current table and creates a new one.
System log tables can be customized by creating a config file with the same name as the table under /etc/clickhouse-server/config.d/, or setting corresponding elements in /etc/clickhouse-server/config.xml. Elements can be customized are:
database: database the system log table belongs to. This option is deprecated now. All system log tables are under databasesystem.table: table to insert data.partition_by: specify PARTITION BY expression.ttl: specify table TTL expression.flush_interval_milliseconds: interval of flushing data to disk.engine: provide full engine expression (starting withENGINE =) with parameters. This option is contradict withpartition_byandttl. If set together, the server would raise an exception and exit.
An example:
<clickhouse>
<query_log>
<database>system</database>
<table>query_log</table>
<partition_by>toYYYYMM(event_date)</partition_by>
<ttl>event_date + INTERVAL 30 DAY DELETE</ttl>
<!--
<engine>ENGINE = MergeTree PARTITION BY toYYYYMM(event_date) ORDER BY (event_date, event_time) SETTINGS index_granularity = 1024</engine>
-->
<flush_interval_milliseconds>7500</flush_interval_milliseconds>
</query_log>
</clickhouse>
By default, table growth is unlimited. To control a size of a table, you can use TTL settings for removing outdated log records. Also you can use the partitioning feature of MergeTree-engine tables.
Sources of System Metrics
For collecting system metrics ClickHouse server uses:
CAP_NET_ADMINcapability.- procfs (only in Linux).
procfs
If ClickHouse server does not have CAP_NET_ADMIN capability, it tries to fall back to ProcfsMetricsProvider. ProcfsMetricsProvider allows collecting per-query system metrics (for CPU and I/O).
If procfs is supported and enabled on the system, ClickHouse server collects these metrics:
OSCPUVirtualTimeMicrosecondsOSCPUWaitMicrosecondsOSIOWaitMicrosecondsOSReadCharsOSWriteCharsOSReadBytesOSWriteBytes