Does a restart of an Azure Container Instance update the image?

Question

I'm running an Azure Container Instance of a rather large image (~13GB). When I create a new instance it takes around 20 minutes to pull the image from the Azure Registry. When I update the image and then restart the container it also says it's pulling, but it only takes a few seconds. I tested it by changing the console output and it actually seems to update the image, but why is it taking so much less time?

Answer 1

ACI creates containers without you having to care about the underlying infrastructure, however under the hood these containers are still running on hosts. The first time you start your container, unless you are very lucky, it is unlikely that the underlying host has your container image cached and so it has to download the image, which for a large image will take a while.

When you restart a running container, most of the time it will restart on the same host, and so already have the old image cached. To update to the new image it will only need to download the difference, which is quick.