+ * Open the environment in read-only mode. No write operations will be
+ * allowed. LMDB will still modify the lock file - except on read-only
+ * filesystems, where LMDB does not use locks.
+ * <li>#MDB_WRITEMAP
+ * Use a writeable memory map unless MDB_RDONLY is set. This is faster
+ * and uses fewer mallocs, but loses protection from application bugs
+ * like wild pointer writes and other bad updates into the database.
+ * Incompatible with nested transactions.
+ * Do not mix processes with and without MDB_WRITEMAP on the same
+ * environment. This can defeat durability (#mdb_env_sync etc).
+ * <li>#MDB_NOMETASYNC
+ * 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().
+ * <li>#MDB_NOSYNC
+ * 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().
+ * <li>#MDB_MAPASYNC
+ * When using #MDB_WRITEMAP, use asynchronous flushes to disk.
+ * As with #MDB_NOSYNC, a system crash can then corrupt the
+ * database or lose the last transactions. Calling #mdb_env_sync()
+ * ensures on-disk database integrity until next commit.
+ * This flag may be changed at any time using #mdb_env_set_flags().
+ * <li>#MDB_NOTLS
+ * Don't use Thread-Local Storage. Tie reader locktable slots to
+ * #MDB_txn objects instead of to threads. I.e. #mdb_txn_reset() keeps
+ * the slot reseved for the #MDB_txn object. A thread may use parallel
+ * read-only transactions. A read-only transaction may span threads if
+ * the user synchronizes its use. Applications that multiplex many
+ * user threads over individual OS threads need this option. Such an
+ * application must also serialize the write transactions in an OS
+ * thread, since LMDB's write locking is unaware of the user threads.
+ * <li>#MDB_NOLOCK
+ * Don't do any locking. If concurrent access is anticipated, the
+ * caller must manage all concurrency itself. For proper operation
+ * the caller must enforce single-writer semantics, and must ensure
+ * that no readers are using old transactions while a writer is
+ * active. The simplest approach is to use an exclusive lock so that
+ * no readers may be active at all when a writer begins.
+ * <li>#MDB_NORDAHEAD
+ * Turn off readahead. Most operating systems perform readahead on
+ * read requests by default. This option turns it off if the OS
+ * supports it. Turning it off may help random read performance
+ * when the DB is larger than RAM and system RAM is full.
+ * The option is not implemented on Windows.
+ * <li>#MDB_NOMEMINIT
+ * Don't initialize malloc'd memory before writing to unused spaces
+ * in the data file. By default, memory for pages written to the data
+ * file is obtained using malloc. While these pages may be reused in
+ * subsequent transactions, freshly malloc'd pages will be initialized
+ * to zeroes before use. This avoids persisting leftover data from other
+ * code (that used the heap and subsequently freed the memory) into the
+ * data file. Note that many other system libraries may allocate
+ * and free memory from the heap for arbitrary uses. E.g., stdio may
+ * use the heap for file I/O buffers. This initialization step has a
+ * modest performance cost so some applications may want to disable
+ * it using this flag. This option can be a problem for applications
+ * which handle sensitive data like passwords, and it makes memory
+ * checkers like Valgrind noisy. This flag is not needed with #MDB_WRITEMAP,
+ * which writes directly to the mmap instead of using malloc for pages. The
+ * initialization is also skipped if #MDB_RESERVE is used; the
+ * caller is expected to overwrite all of the memory that was
+ * reserved in that case.
+ * This flag may be changed at any time using #mdb_env_set_flags().