The vSphere statistics subsystem collects data on the resource usage of inventory objects. Data on a wide range of metrics is collected at frequent intervals, processed, and archived in the vCenter Server database. You can access statistical information through command-line monitoring utilities or by viewing performance charts in the vSphere Web Client.
Counters and Metric Groups
vCenter Server systems and hosts use data counters to query for statistics. A data counter is a unit of information relevant to a given inventory object or device. Each counter collects data for a different statistic in a metric group. For example, the disk metric group includes separate data counters to collect data for disk read rate, disk write rate, and disk usage. Statistics for each counter are rolled up after a specified collection interval. Each data counter consists of several attributes that are used to determine the statistical value collected.
For a complete list and description of performance metrics, see the vSphere API Reference.
Collection Levels and Collection Intervals
Collection levels determine the number of counters for which data is gathered during each collection interval. Collection intervals determine the time period during which statistics are aggregated, calculated, rolled up, and archived in the vCenter Server database. Together, the collection interval and collection level determine how much statistical data is collected and stored in your vCenter Server database .
Real-time data appears in the performance charts only for hosts and virtual machines that are powered on. Historical data appears for all supported inventory objects, but might be unavailable during certain circumstances.