Category: software

Key knowledge for the Swap collectd plugin

• The collectd plugin Swap official page – https://collectd.org/wiki/index.php/Plugin:Swap
• The Swap plugin options – https://collectd.org/documentation/manpages/collectd.conf.5.shtml#plugin_swap This article relies on the default plugin’s options

  <Plugin swap>
  #       ReportByDevice false
  #       ReportBytes true
  #       ValuesAbsolute true
  #       ValuesPercentage false
  #       ReportIO true
  </Plugin>
  

  All the devices are reported as a single device not per device and bytes and absolute values are used not percentages.

• to enable the Swap plugin, load the plugin with the load directive in /etc/collectd.conf

  LoadPlugin swap
  

• The Swap plugin collects data every 10 seconds.
• swap_value – includes a single Gauge value under swap type – a metric, which value that can go up and down. It is used to count the swap occupancy for the different categories (the category is saved in a tag value of one record, and the categories are free, used and etc.). So there are multiple gauge values with different tags for the different swap categories at a given time. And a second counter under swap_io type
  

  tag key	tag value	description
  host	server hostname	The name of the source this measurement was recorded.
  type	swap | swap_io	swap is the type, which will group the swap usage categories (cached, free, used). The swap_io groups the swap IO usage – how many IO operations are executed (in, out).
  type_instance	swap categories	The categories are cached, free, used.

• A Gauge value – a metric, which value that can go up and down. More on the topic – Data sources.
  

  A GAUGE value is simply stored as-is. This is the right choice for values which may increase as well as decrease, such as temperatures or the amount of memory used.

• A DERIVE value – a metric, in which the change of the value is interesting. For example, it can go up indefinitely and it is important how fast it goes up, there are functions and queries, which will give the user the derivative value.
  

  These data sources assume that the change of the value is interesting, i.e. the derivative. Such data sources are very common with events that can be counted, for example, the number of emails that have been received per second by an MTA since it was started. The total number of emails is not interesting.

• To cross-check the value, the user can use the /proc/swap, /proc/meminfo and /proc/vmstat

  [root@srv ~]# cat /proc/swaps
  Filename                                Type            Size    Used    Priority
  /dev/zram0                              partition       16777212        2533856 -1
  [root@srv ~]# cat /proc/meminfo |egrep -e "^(SwapTotal:|SwapFree:|SwapCached:)"
  SwapCached:       175416 kB
  SwapTotal:      16777212 kB
  SwapFree:       14202396 kB
  [root@srv ~]# cat /proc/vmstat |egrep -e "^(pswpout|pswpin)"
  pswpin 877611681
  pswpout 376878365
  

  The swap_io values are multiplied by the page size of the current system. For Linux, it is 4K. Note, by default, the ReportBytes collectd option is not enabled, so the swap_io measurement is in pages since the last reboot. The swap_io counter is read from the pswpout and pswpin (i.e. they also represent the pages since the last reboot). In fact, these two values are really important to track down because they tell how much the system touches swap device(s) and it could point out a problem with the physical memory shortages.