The practice of buffering to a tempfile during a pushing contributes massively
to slow V2 push performance perception. The protocol was actually designed to
avoid precalculation, supporting cut-through data push. This means we can
assemble the layer, calculate its digest and push to the remote endpoint, all
at the same time.
This should increase performance massively on systems with slow disks or IO
bottlenecks.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
