Don't flush system buffers to disk when committing a transaction. This optimization means a system crash can corrupt the database or lose the last transactions if buffers are not yet flushed to disk. The risk is governed by how often the system flushes dirty buffers to disk and how often (mdb-env-sync) is called. However, if the filesystem preserves write order and the MDB_WRITEMAP flag is not used, transactions exhibit ACI (atomicity, consistency, isolation) properties and only lose D (durability). I.e. database integrity is maintained, but a system crash may undo the final transactions. Note that (MDB_NOSYNC | MDB_WRITEMAP) leaves the system with no hint for when to write transactions to disk, unless (mdb-env-sync) is called. (MDB_MAPASYNC | MDB_WRITEMAP) may be preferable. This flag may be changed at any time using (mdb-env-set-flags).