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 tables metric_log, query_log, query_thread_log, trace_log are served by MergeTree 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 To control a size of a table, you can use TTL settings for removing outdated log records.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_ADMIN capability.
  • procfs (only in Linux).

procfs

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).

If procfs is supported and enabled on the system, ClickHouse server collects these metrics:

  • OSCPUVirtualTimeMicroseconds
  • OSCPUWaitMicroseconds
  • OSIOWaitMicroseconds
  • OSReadChars
  • OSWriteChars
  • OSReadBytes
  • OSWriteBytes

Rating: 5 - 2 votes

Was this content helpful?
★★★★★