From the README:

FileDB is a Zig-implementation of Bitcask by Riak paper.

• FileDB stores record metadata in a log-structured hashtable and parallely keeps 1 disk file open for inserting records in append-only mode. On restarts or MAX_FILE_REACHED, the disk file is rotated and all the oldfiles are kept open for reading only.
• A compaction process running every config.compactionInterval seconds, reads all the disk files and combines them into one file while updating the metadata hashtable.
• A sync process syncs the open disk files once every config.syncInterval. Sync also can be done on every request if config.alwaysFsync is True.

Read about internals in-depth at FileDb.