ITS#7725 add MDB_NORDAHEAD flag for env_open

libraries/liblmdb/lmdb.h

@@ -275,6 +275,8 @@ typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *rel
 #define MDB_NOTLS		0x200000
 	/** don't do any locking, caller must manage their own locks */
 #define MDB_NOLOCK		0x400000
+	/** don't do readahead (no effect on Windows) */
+#define MDB_NORDAHEAD	0x800000
 /** @} */
 
 /**	@defgroup	mdb_dbi_open	Database Flags
@@ -538,6 +540,12 @@ int  mdb_env_create(MDB_env **env);
 	 *		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.
 	 * </ul>
 	 * @param[in] mode The UNIX permissions to set on created files. This parameter
 	 * is ignored on Windows.

libraries/liblmdb/mdb.c

@@ -3283,14 +3283,17 @@ mdb_env_map(MDB_env *env, void *addr, int newsize)
 		env->me_map = NULL;
 		return ErrCode();
 	}
-	/* Turn off readahead. It's harmful when the DB is larger than RAM. */
+
+	if (flags & MDB_NORDAHEAD) {
+		/* Turn off readahead. It's harmful when the DB is larger than RAM. */
 #ifdef MADV_RANDOM
-	madvise(env->me_map, env->me_mapsize, MADV_RANDOM);
+		madvise(env->me_map, env->me_mapsize, MADV_RANDOM);
 #else
 #ifdef POSIX_MADV_RANDOM
-	posix_madvise(env->me_map, env->me_mapsize, POSIX_MADV_RANDOM);
+		posix_madvise(env->me_map, env->me_mapsize, POSIX_MADV_RANDOM);
 #endif /* POSIX_MADV_RANDOM */
 #endif /* MADV_RANDOM */
+	}
 #endif /* _WIN32 */
 
 	/* Can happen because the address argument to mmap() is just a