CRFS is a read-only FUSE filesystem that lets you mount a container image, served directly from a container registry (such as gcr.io), without pulling it all locally first. Go's continuous build system tests Go on many operating systems and architectures, using a mix of containers (mostly for Linux) and VMs (for other operating systems). We prioritize fast builds, targetting 5 minute turnaround for pre-submit tests when testing new changes. For isolation and other reasons, we run all our containers in a single-use fresh VMs. Generally our containers do start quickly, but some of our containers are very large and take a long time to start. To work around that, we've automated the creation of VM images where our heavy containers are pre-pulled. This is all a silly workaround. It'd be much better if we could just read the bytes over the network from the right place, without the all the hoops.