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Prior to starting a virtual machine, the virtual machine image used must be transferred to the compute node from the Image Service. How this works can change depending on the settings chosen for the compute node and the Image Service. Typically, the Compute service will use the image identifier passed to it by the scheduler service and request the image from the Image API. Though images are not stored in glance—rather in a back end, which could be Object Storage, a filesystem or any other supported method—the connection is made from the compute node to the Image Service and the image is transferred over this connection. The Image Service streams the image from the back end to the compute node. Certain back ends support a more direct method, where on request the Image Service will return a URL that can be used to download the image directly from the back-end store. Currently the only store to support the direct download approach is the filesystem store. It can be configured using the filesystems option in the image_file_url section of the nova.conf file on compute nodes. Compute nodes also implement caching of images, meaning that if an image has been used before it won't necessarily be downloaded every time. Information on the configuration options for caching on compute nodes can be found in the Configuration Reference.