Flush system buffers to disk only once per transaction, omit the metadata flush. Defer that until the system flushes files to disk, or next non-MDB_RDONLY commit or (mdb-env-sync). This optimization maintains database integrity, but a system crash may undo the last committed transaction. I.e. it preserves the ACI (atomicity, consistency, isolation) but not D (durability) database property. This flag may be changed at any time using (mdb-env-set-flags).