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fix(#2264): boot.tmp.useTmpfs = false — disk-backed /tmp for agents

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Agent containers currently mount /tmp as a 3.2 GB tmpfs (RAM-backed).
Large scratch writes (nix-develop shells, cargo build dirs) eat container
RAM unnecessarily and can exhaust the tmpfs, losing in-flight work.

Set boot.tmp.useTmpfs = false so /tmp is disk-backed instead.
boot.tmp.cleanOnBoot (default true) preserves the ephemeral-per-boot
semantics — /tmp is cleared on container start, same as before.

Closes #2264. Supersedes PR #2299 (TMPDIR redirect + state/tmp).
This commit is contained in:
atlas 2026-07-10 02:46:25 +02:00 • committed by mara
commit 6da7835ad9
1 changed files with 13 additions and 0 deletions
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13
nix/templates/harness-base.nix
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@ -1442,6 +1442,19 @@ in
boot.isNspawnContainer = true; boot.isNspawnContainer = true;
# Use a disk-backed /tmp instead of the default tmpfs so large scratch
# writes (nix-develop shells, cargo build dirs, multi-GB downloads) land
# on disk rather than eating container RAM. The tmpfs default mounts
# ~3.2 GB of RAM per container; disk-backed /tmp is effectively unlimited
# and cheaper for agents that do heavy build work.
#
# cleanOnBoot defaults to false in nixpkgs — set it explicitly so /tmp is
# cleared on each container start (D! tmpfiles rule), preserving the same
# ephemeral-per-boot semantics agents already expect from the old tmpfs,
# just without the RAM cost.
boot.tmp.useTmpfs = false;
boot.tmp.cleanOnBoot = true;
# Every agent gets flakes + the modern `nix` CLI out of the box. # Every agent gets flakes + the modern `nix` CLI out of the box.
# Equivalent to passing `--extra-experimental-features 'nix-command # Equivalent to passing `--extra-experimental-features 'nix-command
# flakes'` on every invocation. Agents shell out to `nix build` / # flakes'` on every invocation. Agents shell out to `nix build` /