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What is collectd?

  • collectd is a system statistics collection daemon; it uses plugins to collect system statistics and can publish those statistics in a number of ways.
  • Free open source project, GNU General Public License, version 2 (GPLv2), the remaining files are licensed under other open source licenses
  • collectd also provides some simple thresholding and event notification capabilities

Why use collectd?

  • The collectd API, the selection of available plugins (90+) coupled with the modular nature of collectd architecture, provides a generic path to expose platform statistics from all existing plugins and newly developed plugins to OpenStack or other fault management applications.

  • collectd also has bindings for several programming languages, which allows a developer to implement a plug-in in their language of choice, without having to modify the receiver of the collected metrics, for example Ceilometer itself.

collectd architecture

  • plugins in collectd fall under one of the following categories:
  • Input plugins: read system statistics at a regular interval, and dispatch the values to the collectd daemon.
    • Output plugins: receive dispatched values from the daemon and output/write/dispatch those values in various output formats (e.g. JSON, SNMP, AMQP, MySQL, HTTP, etc).
    • Binding plugins: which provide language bindings for collectd to allow plugins to be written in languages other than C.
    • Logging plugins: which write information to log files or dispatch messages to syslog.
    • Notification plugins: enable limited monitoring support in collectd.
    • Other plugins which can both read and dispatch values.

collectd data flow

collectd statistics: 

Statistics in collectd consist of a value list. A value list includes:

  • Values
  • Value length: the number of values in the data set.
  • Time: timestamp at which the value was collected.
  • Interval: interval at which to expect a new value.
  • Host: used to identify the host.
  • Plugin: used to identify the plugin.
  • Plugin instance (optional): used to group a set of values together. For e.g. values belonging to a DPDK interface.
  • Type: unit used to measure a value. In other words used to refer to a data set.
  • Type instance (optional): used to distinguish between values that have an identical type.
  • meta data: an opaque data structure that enables the passing of additional information about a value list. “Meta data in the global cache can be used to store arbitrary information about an identifier” 

    Host, plugin, plugin instance, type and type instance uniquely identify a collectd value

collectd Values

Values, can be one of:
  • Derive: used for values where a change in the value since it’s last been read is of interest. Can be used to calculate and store a rate.
  • Counter: similar to derive values, but take the possibility of a counter wrap around into consideration.
  • Gauge: used for values that are stored as is.
  • Absolute: used for counters that are reset after reading collectd consist of a value list. 

 

collectd Data Sets

Values lists are often accompanied by data sets that describe the values in more detail. Data sets consist of:

  • A type: a name which uniquely identifies a data set.
  • One or more data sources (entries in a data set) which include:
  • The name of the data source. If there is only a single data source this is set to “value”.
  • The type of the data source, one of: counter, gauge, absolute or derive.
  • A min and a max value.

Examples of types in collectd

Examples of types in types.db:

bitrate value:GAUGE:0:4294967295 
counter value:COUNTER:U:U
if_octets rx:COUNTER:0:4294967295,tx:COUNTER:0:4294967295 

 

In the example above if_octets has two data sources: tx and rx.

 

collectd notifications

Notifications in collectd are generic messages containing:

  • An associated severity, which can be one of OKAY, WARNING, and FAILURE.
  • A time.
  • A Message
  • A host.
  • A plugin.
  • A plugin instance (optional).
  • A type.
  • A types instance (optional).
  • Meta-data.

Notifications and thresholds

 

 

References

https://collectd.org/

https://collectd.org/faq.shtml

https://collectd.org/documentation.shtml

https://collectd.org/wiki/index.php/Notifications_and_thresholds 

https://collectd.org/wiki/index.php/First_steps

https://collectd.org/wiki/index.php/Plugin_architecture

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