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nixos/qemu-vm: introduce virtualisation.mountHostNixStore option

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Now that `useBootLoader` produces a full system image, moving disk
images can be slow because they have a full Nix store in them.

It does not make sense to keep the 9p mountpoint to shadow the
/nix/store of the VM.

We disable it if we have `useBootLoader` and introduce an option for
easy overrides.
This commit is contained in:
Raito Bezarius 2023-04-23 23:28:54 +02:00
parent 6d69171610
commit 0df5257b82
1 changed files with 20 additions and 2 deletions
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22
nixos/modules/virtualisation/qemu-vm.nix
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@ -574,12 +574,15 @@ in
virtualisation.writableStore =
mkOption {
type = types.bool;
default = true; # FIXME
default = cfg.mountHostNixStore;
defaultText = literalExpression "cfg.mountHostNixStore";
description =
lib.mdDoc ''
If enabled, the Nix store in the VM is made writable by
layering an overlay filesystem on top of the host's Nix
store.
By default, this is enabled if you mount a host Nix store.
'';
};
@ -713,6 +716,21 @@ in
For applications which do a lot of reads from the store,
this can drastically improve performance, but at the cost of
disk space and image build time.
As an alternative, you can use a bootloader which will provide you
with a full NixOS system image containing a Nix store and
avoid mounting the host nix store through
{option}`virtualisation.mountHostNixStore`.
'';
};
virtualisation.mountHostNixStore =
mkOption {
type = types.bool;
default = !cfg.useNixStoreImage && !cfg.useBootLoader;
defaultText = literalExpression "!cfg.useNixStoreImage && !cfg.useBootLoader";
description = lib.mdDoc ''
Mount the host Nix store as a 9p mount.
'';
};
@ -933,7 +951,7 @@ in
virtualisation.additionalPaths = [ config.system.build.toplevel ];
virtualisation.sharedDirectories = {
nix-store = mkIf (!cfg.useNixStoreImage) {
nix-store = mkIf cfg.mountHostNixStore {
source = builtins.storeDir;
target = "/nix/store";
};