Vector keeps information on the internal state of the database in the Write-Ahead Log (WAL), which is stored in the wal directory. The wal directory exists for each database and is located in the database directory of the default data location on HDFS. The wal directory contains the main.wal file and the pdts subdirectory. The pdts subdirectory contains per-partition wal files that store PDTs.

The WAL is similar to that used by other database systems, but also holds other critical information.