Data Granularity and Time-Series Metric Data
The Service Monitor and Host Monitor store time-series metric data in a variety of ways.
When the data is received, it is written as-is to the metric store. Over time, the raw data is summarized to and stored at various data granularities. For example, after ten minutes, a summary point is written containing the average of the metric over the period as well as the minimum, the maximum, the standard deviation, and a variety of other statistics. This process is summarized to produce hourly, six-hourly, daily, and weekly summaries. This data summarization procedure applies only to metric data. When the Impala query and YARN application monitoring storage limit is reached, the oldest stored records are deleted.
The Service Monitor and Host Monitor internally manage the amount of overall storage space dedicated to each data granularity level. When the limit for a level is reached, the oldest data points at that level are deleted. Metric data for that time period remains available at the lower granularity levels. For example, when an hourly point for a particular time is deleted to free up space, a daily point still exists covering that hour. Because each of these data granularities consumes significantly less storage than the previous summary level, lower granularity levels can be retained for longer periods of time. With the recommended amount of storage, weekly points can often be retained indefinitely.
Some features, such as a detailed display of health results, depend on the presence of raw data. Cluster utilization reports depend on hourly data being available for the selected time range. Charts built from weekly data might show a large gap between the last data point and the current time, as the next data point is not available yet. Other granularity levels may exhibit a similar gap, due in part to delays in the summarization process. These gaps may coincide with another one on the other side of the chart if metric history is too short to cover the selected time range. Health history is maintained by the event store dictated by its retention policies.