Velox supports three data serialization formats that can be used for data shuffle: PrestoPage, UnsafeRow and CompactRow. PrestoPage is a columnar format. UnsafeRow and CompactRow are row-wise formats.
Velox applications can register their own formats as well.
PrestoPage format is described in the Presto documentation.
UnsafeRow format comes from Apache Spark.
CompactRow is similar to UnsafeRow, but it is more space efficient and results in fewer bytes shuffled which has a cascading effect on CPU usage (for compression and checksumming) and memory (for buffering).
The details of UnsafeRow and CompactRow formats can be found in the following articles.
Velox also uses another row-wise serialization format, ContainerRowSerde, for storing data in aggregation and join operators. This format is similar to CompactRow.