文件系统缓存

此插件将地形图块、特征向量和其他数据缓存到文件夹层次结构中的本地文件系统。每个缓存的数据元素都在一个单独的文件中，并且可能包含一个关联的元数据文件。

示例用法：

<map>
        <options>
        <cache driver="filesystem">
                <path>c:/osgearth_cache</path>
        </cache>
                    ...

笔记：：

The ``filesystem`` cache stores each class of data in its own ``bin``.
    Each ``bin`` has a separate directory under the root path. osgEarth
    controls the naming of these bins, but you can use the ``cache_id``
    property on map layers to customize the naming to some extent.

    This cache supports expiration, but does NOT support size limits --
    there is no way to cap the size of the cache.

    Cache access is serialized since we are reading and writing
    individual files on disk.

    Accessing the cache from more than one process at a time may cause
    corruption.

    The actual format of cached data files is "black box" and may change
    without notice. We do not intend for cached files to be used directly
    or for other purposes.

性能：

路径

存储所有缓存箱和文件的根目录的位置。