If you are gonna enable M53 as a stepping stone (i.e., everybody needs to update to M53 first, reboot and then take the latest update) I have some good news for you!

You can switch full payload generation to use payload v2 (XZ compression instead of bz2), which reduces the size of the full payload by quite a bit (I recall something like 15%). This has a few implications:

* old devices can *not* apply a v2 full payload; which is not a problem because you would generate all full payloads in v1 format for target version <= M53 (you already generated them all, but still if you wish to regenerate one we can easily make the M53 be v1)

* xz compression in the delta_generator needs to be implemented (the one in Android uses a different XZ library not available in CrOS that collides with the name of the one in chrome). This should be easy to do, just requires some ebuild dep and some glue code to call the lib to compress a chunk:
https://chromium.googlesource.com/aosp/platform/system/update_engine/+/master/payload_generator/xz_chromeos.cc

For deltas, we could use xz as well whenever supported (automatically detected), but as I said that function is just not implemented.

just saying... 

Oh, I thought we already used XZ for deltas, when the client supported it.

Unrelated to the stepping zone, ChromeOS xz support in the delta_generator seems like a smart idea. Can we get an owner for fixing that?

Client support for XZ landed in... hmm... 7724.0.0 (any client that declares minor-version 3 support, which is the latest minor-version until we add puffin)

https://chromium-review.googlesource.com/c/316912/

Yes, but can we get an owner for delta_generator support?

Closing this bug since we now have deltas for the M53 stepping stones

I have a filed issue 710154 for the XZ support implementation

So.... we have an R53 stepping stone.

Do we have a hard commitment to maintain it forever?