From: Scott Worley Date: Thu, 22 Jun 2023 22:10:39 +0000 (-0700) Subject: New runTest interface fixes useNixStoreImage test X-Git-Url: http://git.scottworley.com/nixos-qemu-vm-isolation/commitdiff_plain/47cb654f45831f96a26819d1fcb39770662cd579?ds=sidebyside New runTest interface fixes useNixStoreImage test The new runTest interface lets us turn off includeTestScriptReferences, which lets the useNixStoreImage test pass. Unfortunately, I then discovered another problem with useNixStoreImage: It builds a new store image every time the VM starts up! This is undesirable for two reasons: 1. It significantly delays VM start-up. For network-service-providing VMs, this is downtime. Longer startup time likely won't trouble high-availability services with fancy load balancers, health checks, and rolling restarts, but for unsophisticated use cases that currently just accept some small amount of downtime during VM restarts for software updates, using useNixStoreImage means much longer periods of interrupted service. 2. This wastes disk I/O, building the same image over and over. This useNixStoreImage design choice was made in order to keep the guest's Nix store image out of the host's Nix store. This is especially important for the public Hydra that populates the public binary cache that's currently hosted on centralized pay-per-byte commercial storage services -- it doesn't make sense to pay to store and host the Nix store itself and also many little copies of overlapping pieces of it. So here in nixos-qemu-vm-isolation, we're happy to pay the cost of storing the guests' Nix store images in the host's Nix store, because we really value the faster start-up, and because we don't have the problem of large storage service bills and no budget to pay them with. :) --- diff --git a/README.md b/README.md index c0aea31..ec9ba70 100644 --- a/README.md +++ b/README.md @@ -3,5 +3,6 @@ squashfs for the VM's /nix/store that contains only the VM's dependencies (like the installer has) rather than a virtio mount of the host's entire /nix/store. -**Update:** `virtualisation.useNixStoreImage` exists now! But it doesn't -work! :( See the note in `checks/mount-grep.nix` +**Update:** `virtualisation.useNixStoreImage` exists now! But it builds +the store image at runtime, rather that at build-time, drastically +increasing VM start-up time. diff --git a/checks/mount-grep.nix b/checks/mount-grep.nix index 90ae4a7..33478ea 100644 --- a/checks/mount-grep.nix +++ b/checks/mount-grep.nix @@ -3,6 +3,8 @@ pkgs: { hostPkgs = pkgs; + includeTestScriptReferences = false; + nodes = { shared = _: { }; private = _: { imports = [ ../modules/qemu-vm-isolation.nix ]; }; @@ -26,8 +28,6 @@ pkgs: { shared.succeed("[[ -e ${pkgs.pv} ]]") private.fail("[[ -e ${pkgs.pv} ]]") - - # useNixStoreImage isn't ready until this works: - # useNixStoreImage.fail("[[ -e ${pkgs.pv} ]]") + useNixStoreImage.fail("[[ -e ${pkgs.pv} ]]") ''; }