Bug 180189 Support in memory data set cache.
Currently, Design-Time Data Cache is stored on disks. By performance tests using ramdisk, we found that caching in physical memory can improve the performance of RunAndRender by 30%.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/api/DataEngineContext.java
if ( outputStream == null )
				throw new DataException( ResourceConstants.RD_SAVE_STREAM_ERROR );
			return outputStream;
		}
		catch (IOException e)
		{
			throw new DataException( ResourceConstants.RD_SAVE_STREAM_ERROR, e );
		}
	}
	/**
	 * Determins whether one particular stream exists
	 * 
	 * @param streamID
	 * @param subStreamID
	 * @param streamType
	 * @return boolean value
	 */
	public boolean hasOutStream( String streamID, String subStreamID,
			int streamType )
	{
		String relativePath = getPath( streamID, subStreamID, streamType );
		if ( writer != null )
			return writer.exists( relativePath );
		else
			return false;
	}
	
	/**
	 * @param streamID
	 * @param subStreamID