In order to offer good read and write performance, the Cassandra PV Archiver arranges the data in a way that is optimized for the Cassandra database. Cassandra tables organize rows in so-called partitions. A partition is a set of rows that is stored on the same node. While the rows within a partition have an order (and thus range queries are possible), there is no order between partitions.
When storing time-series data, this means that only data in the same partition can easily be queried for a certain period of time. Unfortunately, storing all data for a certain channel in a single partition is typically not an option because the size of a partition should typically not exceed 100 MB in order to attain a good performance .
The Cassandra PV Archiver solves this issue by dividing the data stored for each channel into so-called sample buckets (see Figure I.2, “Division of samples into sample buckets”). Each sample bucket stores the data for a certain period of time. When a sample buckets hits a size of about 100 MB, a new sample bucket is started. The information about how periods of time map to sample buckets is stored in a separate table. When reading data, the Cassandra PV Archiver first finds out which sample buckets exist for the specified period of time and then retrieves the actual data from these sample buckets.
By default, Apache Cassandra compresses data before writing it to disk. For this reason, the on-disk size of a sample bucket is typically significantly less than 100 MB. However, the 100 MB limit recommended for partitions applies to the uncompressed size.
Typically, an administrator or developer does not have to deal with these details of how data is stored. However, it is important to understand these details when optimizing the configuration of the Cassandra database cluster for performance and when reading data directly from the database, bypassing the query interface provided by the Cassandra PV Archiver.
Each control-system support uses a separate table (or possibly even a set of tables) for storing its samples. However, the control-system support does not have to deal with managing sample buckets. When writing a sample, the Cassandra PV Archiver tells the control-system support to which sample bucket a sample belongs. This way, the control-system support can simply store the sample in this sample bucket. In the same way, when reading data, the Cassandra PV Archiver only asks the control-system support for data from a single sample bucket, so that the control-system support can use simple range queries.