make-disk-image: add compressed option to compress qcow2 images #31535
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You probably think about installation media images (.iso). Virtual machine images (.qcow2, .ami) are meant to be mutable, this is why they contain uncompressed /nix/store. (In fact virtualization software often keeps the original image unmodified by storing writes performed by the guest operating system in a separate file, while all that a guest sees is a single writable disk. In this case it is perfect to keep the read only disk in the form that is compressed and supports fast random read access, and qcow2 compression provides exactly this.) |
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Sure, I've noticed that which is why I've deleted the comment. Didn't know about storing writes separately, interesting, makes sense to deduplicate this given instances are meant to be ephemeral and usually don't get heavy updates.
At least Amazon doesn't support qcow2 at all: their import tool supports raw and vpc, though. |
nixos/lib/make-disk-image.nix
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| @@ -45,7 +49,9 @@ let | |||
| raw = "img"; | |||
| }; | |||
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| nixpkgs = lib.cleanSource pkgs.path; | |||
| compress = optionalString compressed (assert format == "qcow2"; "-c"); | |||
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Rather than putting an assert here, maybe we could just make format be one of vpc, qcow2, qcow2-compressed, and raw? From a types perspective it feels cleaner to have a 4-element enum than a boolean paired with a 3-element enum where 2 of the choices are invalid.
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This makes the naming difficult: we shouldn't rename format parameter as it is a part of the API, and there is no better name for the actual format as understood by qemu-img. Is this better: 27f5cc9?
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An alternative solution is to rename compressed into qcow2-compressed:
- compress = optionalString compressed (assert format == "qcow2"; "-c");
+ compress = optionalString (format == "qcow2" && qcow2-compressed) "-c";(This facilitates making qcow2 be compressed by default, although I'm not sure if that is useful.)
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I quite like the qcow2-compressed version myself. Thanks!
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This is |
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Sorry for not testing the final version thoroughly enough. Fixed in 470a775. |
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Now Hydra's repeatedly getting |
Motivation for this change
Compressed images take less space (about 3 times less), are faster to upload and to download to provision new instances. I'm interested in using this with qemu and OpenStack. I'm not going to test if compressed images are supported by Amazon or other cloud providers.
The commits are easier to review separately.
Things done
build-use-sandboxinnix.confon non-NixOS)compresseddefaulted totruenix-shell -p nox --run "nox-review wip"./result/bin/)