bool AsynchronousSync { set; }

turns asynchronous sync on and off.

One of the most costly operations during commit is the call to flush the buffers of the database file. In regular mode the commit call has to wait until this operation has completed. When asynchronous sync is turned on, the sync operation will run in a dedicated thread, blocking all other file access until it has completed. This way the commit call can return immediately. This will allow db4o and other processes to continue running side-by-side while the flush call executes. Use this setting with care: It means that you can not be sure when a commit call has actually made the changes of a transaction durable (flushed through OS and file system buffers). The latency time until flushing happens is extremely short. The dedicated sync thread does nothing else except for calling sync and writing the header of the database file when needed. A setup with this option still guarantees ACID transaction processing: A database file always will be either in the state before commit or in the state after commit. Corruption can not occur. You can just not rely on the transaction already having been applied when the commit() call returns.