ClickHouse/docs/en/operations/system-tables/index.md

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---
toc_priority: 52
toc_title: System Tables
---
# System Tables {#system-tables}
## Introduction {#system-tables-introduction}
System tables provide information about:
- Server states, processes, and environment.
- Servers internal processes.
System tables:
- Located in the `system` database.
- Available only for reading data.
- Cant 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.
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Unlike other system tables, the system tables [metric_log](../../operations/system-tables/metric_log.md#system_tables-metric_log), [query_log](../../operations/system-tables/query_log.md#system_tables-query_log), [query_thread_log](../../operations/system-tables/query_thread_log.md#system_tables-query_thread_log), [trace_log](../../operations/system-tables/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.
By default, table growth is unlimited. To control a size of a table, you can use [TTL](../../sql-reference/statements/alter/ttl.md#manipulations-with-table-ttl) settings for removing outdated log records. Also you can use the partitioning feature of `MergeTree`-engine tables.
## Sources of System Metrics {#system-tables-sources-of-system-metrics}
For collecting system metrics ClickHouse server uses:
- `CAP_NET_ADMIN` capability.
- [procfs](https://en.wikipedia.org/wiki/Procfs) (only in Linux).
**procfs**
If ClickHouse server doesnt 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:
- `OSCPUVirtualTimeMicroseconds`
- `OSCPUWaitMicroseconds`
- `OSIOWaitMicroseconds`
- `OSReadChars`
- `OSWriteChars`
- `OSReadBytes`
- `OSWriteBytes`
[Original article](https://clickhouse.tech/docs/en/operations/system-tables/) <!--hide-->