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50 lines
2.2 KiB
Markdown
50 lines
2.2 KiB
Markdown
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---
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toc_priority: 52
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toc_title: System Tables
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---
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# System Tables {#system-tables}
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## Introduction {#system-tables-introduction}
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System tables provide information about:
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- Server states, processes, and environment.
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- Server’s internal processes.
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System tables:
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- Located in the `system` database.
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- Available only for reading data.
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- Can’t be dropped or altered, but can be detached.
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Most of system tables store their data in RAM. A ClickHouse server creates such system tables at the start.
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Unlike other system tables, the system tables [metric\_log](metric_log.md#system_tables-metric_log), [query\_log](query_log.md#system_tables-query_log), [query\_thread\_log](query_thread_log.md#system_tables-query_thread_log), [trace\_log](trace_log.md#system_tables-trace_log) are served by [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) table engine and store their data in a storage filesystem. 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.
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By default, table growth is unlimited. To control a size of a table, you can use [TTL](../../sql-reference/statements/alter.md#manipulations-with-table-ttl) settings for removing outdated log records. Also you can use the partitioning feature of `MergeTree`-engine tables.
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## Sources of System Metrics {#system-tables-sources-of-system-metrics}
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For collecting system metrics ClickHouse server uses:
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- `CAP_NET_ADMIN` capability.
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- [procfs](https://en.wikipedia.org/wiki/Procfs) (only in Linux).
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**procfs**
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If ClickHouse server doesn’t have `CAP_NET_ADMIN` capability, it tries to fall back to `ProcfsMetricsProvider`. `ProcfsMetricsProvider` allows collecting per-query system metrics (for CPU and I/O).
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If procfs is supported and enabled on the system, ClickHouse server collects these metrics:
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- `OSCPUVirtualTimeMicroseconds`
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- `OSCPUWaitMicroseconds`
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- `OSIOWaitMicroseconds`
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- `OSReadChars`
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- `OSWriteChars`
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- `OSReadBytes`
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- `OSWriteBytes`
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[Original article](https://clickhouse.tech/docs/en/operations/system-tables/) <!--hide-->
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