Support passing previous image manifest(s) to influence packing

[jlebon]

This is something that rpm-ostree does and makes sense to do here too. In setups like FCOS where the update cadence is slower, this should make a huge difference in layer efficiency.

[jlebon]

A preparatory step for this is to enhance component names to fully identify the layer contents. Otherwise it's impossible to reliably tell which layers from the previous image can actually be reused.

[jlebon]

I think also this has a wider scope than just having good deltas against a previous image, but also when doing derived builds. E.g. if you're adding content on top of a base image with content-based layers, you may want to do content-based splitting. But without the RFE here, any resplitting may trivially "contaminate" the lower base layers, reducing the good delta you'd normally inherit from base updates.

Could be interesting also to support a mode where you rechunk just those "top" layers and leave the base layers alone.

[cgwalters]

In #97 you say

The reason that hasn't been a priority for me is that most users I think don't have clear update/build cadances like FCOS/OCP does so the focus is on optimizing for good deltas in general.

I don't think that's true; a weekly/monthly patching cadence is a pretty common baseline I hear.

[jlebon]

I don't think that's true; a weekly/monthly patching cadence is a pretty common baseline I hear.

Worth teasing apart "update cadence" from "build cadence" here. The trend is definitely more towards rebuilding and pushing everyday or even faster (all of Fedora minus FCOS), bootc images, UBI, Hummingbird, etc... There's been talk in the past to have slower update cadence for the Atomic desktops for better deltas but I haven't heard much on that topic in a while either. Perhaps the appetite is no longer there either.

Edit: but anyway, yes your point is taken that a lot of people/orgs do truly have well-defined and spaced release cadences. I don't have a measure of what's more common than what so I shouldn't have said "most users" there. Regardless we do want this RFE eventually!