Apache Cassandra™ 1.2

How compaction and compression affect reads

To prevent read speed from deteriorating, compaction runs in the background without random I/O. Compression maximizes the storage capacity of nodes and reduces disk I/O, particularly for read-dominated workloads.

When I/O activity starts to increase in Cassandra due to increased read load, typically the remedy is to add more nodes to the cluster. Cassandra avoids decompressing data in the middle of reading a data file, making its compression application-transparent.