Vantage uses a Write Ahead Logging (WAL) protocol. According to this protocol, permanent data is written to a log file of update records. The log file is written to disk at important times, such as at transaction commit.
Updates to permanent data from different transactions, written to the WAL log, can also be batched. Batching significantly reduces I/O write operations. One I/O operation can represent multiple updates to permanent data.
The WAL Log is conceptually similar to a table, but has a simpler structure. Log data is a sequence of WAL records, different from normal row structure and not accessible by SQL.
- Redo Records for updating disk blocks and insuring file system consistency during restarts, based on operations performed in cache during normal operation.
- Transient Journal (TJ) records used for transaction rollback.
WAL protects permanent tables and system tables, except Transient Journal (TJ) tables, user journal tables, and restartable spool tables (global temporary tables). If the system fails, you can reconstruct Vantage from the WAL log.
The file system stages in-place writes through a disk area called the DEPOT, a collection of cylinders. Staging in-place writes through the DEPOT ensures that either the old or the new copy is available after a restart.