mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-09-20 16:50:48 +00:00
82 lines
4.2 KiB
Markdown
82 lines
4.2 KiB
Markdown
---
|
||
slug: /en/operations/system-tables/
|
||
sidebar_position: 52
|
||
sidebar_label: Overview
|
||
pagination_next: 'en/operations/system-tables/asynchronous_metric_log'
|
||
---
|
||
|
||
# System Tables
|
||
|
||
## Introduction {#system-tables-introduction}
|
||
|
||
System tables provide information about:
|
||
|
||
- Server states, processes, and environment.
|
||
- Server’s internal processes.
|
||
|
||
System tables:
|
||
|
||
- Located in the `system` database.
|
||
- 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](../../operations/system-tables/metric_log.md), [query_log](../../operations/system-tables/query_log.md), [query_thread_log](../../operations/system-tables/query_thread_log.md), [trace_log](../../operations/system-tables/trace_log.md), [part_log](../../operations/system-tables/part_log.md), [crash_log](../../operations/system-tables/crash-log.md) and [text_log](../../operations/system-tables/text_log.md) are served by [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) 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 database `system`.
|
||
- `table`: table to insert data.
|
||
- `partition_by`: specify [PARTITION BY](../../engines/table-engines/mergetree-family/custom-partitioning-key.md) expression.
|
||
- `ttl`: specify table [TTL](../../sql-reference/statements/alter/ttl.md) expression.
|
||
- `flush_interval_milliseconds`: interval of flushing data to disk.
|
||
- `engine`: provide full engine expression (starting with `ENGINE =` ) with parameters. This option is contradict with `partition_by` and `ttl`. If set together, the server would raise an exception and exit.
|
||
|
||
An example:
|
||
|
||
```xml
|
||
<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](../../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 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:
|
||
|
||
- `OSCPUVirtualTimeMicroseconds`
|
||
- `OSCPUWaitMicroseconds`
|
||
- `OSIOWaitMicroseconds`
|
||
- `OSReadChars`
|
||
- `OSWriteChars`
|
||
- `OSReadBytes`
|
||
- `OSWriteBytes`
|
||
|
||
## Related content
|
||
|
||
- Blog: [System Tables and a window into the internals of ClickHouse](https://clickhouse.com/blog/clickhouse-debugging-issues-with-system-tables)
|
||
- Blog: [Essential monitoring queries - part 1 - INSERT queries](https://clickhouse.com/blog/monitoring-troubleshooting-insert-queries-clickhouse)
|
||
- Blog: [Essential monitoring queries - part 2 - SELECT queries](https://clickhouse.com/blog/monitoring-troubleshooting-select-queries-clickhouse)
|
||
|