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[RFC 0087] Promote aarch64-linux to Tier 1 support #87

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[RFC 0087] Promote aarch64-linux to Tier 1 support #87

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---
feature: aarch64-tier1
start-date: 2021-03-09
author: Vika Shleina
co-authors: TBD
shepherd-team: TBD
shepherd-leader: TBD
related-issues: TBD
---

# Summary
[summary]: #summary

Move `aarch64-linux` from a Tier 2 platform to Tier 1, as described in [RFC 0046](/rfcs/0046-platform-support-tiers.md)

# Motivation
[motivation]: #motivation

`aarch64-linux` support in Nixpkgs and NixOS matures over time and becomes
more and more stable, and more devices appear having NixOS on ARM support.
Moving it to a Tier 1 platform will allow us to block release channels on
aarch64-related build failures, making it easier and safer for ARM users
to upgrade their systems, and will help in keeping software versions in
sync between several architectures due to `x86_64-linux` and `aarch64-linux`
builds sharing a channel.

`aarch64-linux` will benefit from increased binary cache coverage as an
additional result, saving on build times for end users.
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# Detailed design
[design]: #detailed-design

If this RFC is accepted, `aarch64-linux` builds will be added to stable
and unstable channels' `tested` aggregate jobs on Hydra, giving them ability
to block channel advances. Hydra will start building aarch64 packages and run
aarch64-based tests as part of stable and unstable channels, including them in
the binary cache, increasing its coverage as a result.
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It is possible that the availability of aarch64 builders from Equinix Metal will at times be
reduced, causing delays in aarch64 build capacity. We will extend the
nixos-org-configurations implementation of hydra-provisioner to dynamically allocate
aarch64 builders on AWS during these capacity shortfalls.

# Examples and Interactions
[examples-and-interactions]: #examples-and-interactions

<!-- This section illustrates the detailed design. This section should clarify all
confusion the reader has from the previous sections. It is especially important
to counterbalance the desired terseness of the detailed design; if you feel
your detailed design is rudely short, consider making this section longer
instead. -->

In [nixos/release-combined.nix](https://github.com/NixOS/nixpkgs/blob/master/nixos/release-combined.nix)
`aarch64-linux` will be moved to `supportedSystems`, enabling NixOS tests
to block channel advances in case of failures.

(maybe something more? IDK for now)
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# Drawbacks
[drawbacks]: #drawbacks

Some build failures could unneccesarily delay channel advances, delaying critical updates.

# Alternatives
[alternatives]: #alternatives

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I am of the opinion a separate channel should be the first step. Show there are enough resources and commitment to keep it green for half a year to a year. If that is the case, we can upgrade it to Tier 1. I don't think generally there is enough understanding in how much day-to-day effort goes into actually keeping the channels green. At the same time, I have no idea how few breakage there is with aarch64 nowadays.

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I added your suggestion to the manuscript.

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The channel already blocks on aarch64-linux since late 2018 for the limited support set. So I guess we're ready to upgrade it to Tier 1 since things were kept green for half a year to a year.

It's all about having the jobs being tried to be built. Not about adding new blockers.

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Might be worth specifying which channel we are talking about here. If you mean the unstable channel: it mostly works. If you mean the stable channel: It has been blocked for a few days when I opened the channel PR and probably nobody noticed yet.

Create an aarch64-focused channel that would build same things current `unstable` does, but for aarch64 only.
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# Unresolved questions
[unresolved]: #unresolved-questions

Do we have enough machines to handle aarch64 builds without delaying `x86_64-linux` builds?
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# Future work
[future]: #future-work

TBA