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What is whisper? How is stores data on HDD?

Whisper - is a fixed-size database, similar in design and purpose to RRD (round-robin-database)
Data points in Whisper are stored on-disk as big-endian double-precision floats.
- Each metric is stored in its own file
- Each file has a fixed size, which is regulated by retention policies
- Each data point is stored with its timestamp
- Archives overlap time periods
- All time-slots within an archive take up space whether or not a value is stored

The configuration file storage-schemas.conf details retention rates for storing metrics. It matches metric paths to patterns, and tells whisper what frequency and history of datapoints to store.
storage-schemas.conf

[apache_busyWorkers]
pattern = ^servers\.www.*\.workers\.busyWorkers$
retentions = 15s:7d,1m:21d,15m:5y

This file defines how to aggregate data to lower-precision retentions.
xFilesFactor should be a floating point number between 0 and 1, and specifies what fraction of the previous retention level’s slots must have non-null values in order to aggregate to a non-null value. The default is 0.5.
aggregationMethod specifies the function used to aggregate values for the next retention level. Legal methods are average, sum, min, max, and last. The default is average.
storeage-aggregation.conf

[all_min]
pattern = \.min$
xFilesFactor = 0.1
aggregationMethod = min

terminology
series
- A named set of datapoints. A series is identified by a unique name, which is composed of elements separated by periods (.) which are used to display the collection of series into a hierarchical tree. A series storing system load average on a server called apache02 in datacenter metro_east might be named as:
  - metro_east.servers.apache02.system.load_average
series list
- A series name or wildcard which matches one or more series. Series lists are received by functions as a list of matching series. From a user perspective, a series list is merely the name of a metric. For example, each of these would be considered a single series list:
  - metro_east.servers.apache02.system.load_average.1_min,
  - metro_east.servers.apache0{1,2,3}.system.load_average.1_min
  - metro_east.servers.apache01.system.load_average.*