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Questions regarding impermanence (discuss.tchncs.de)

submitted 2 years ago by ruination@discuss.tchncs.de to c/nixos@lemmy.ml

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I've been reading some articles regarding impermanence in NixOS, particularly this, this, this, and this, and I want to set up impermanence on my desktop system. Since it involves deleting my entire root, however, I figured I'd ask for advice here to learn what the best practices are for doing this to minimise risk of data loss, though it isn't too critical since it's a new setup. For reference, I have a Flake + home-manager setup on ZFS root, though I didn't think to do a snapshot when the disk was empty since I didn't know about impermanence when I started out. I also have separate ZFS datasets for /, /home, /nix, /var, and /var/lib. I want to set up impermanence for both root and home, with some persistent directories on home, but I'm not sure if I should set it up on both at once or if I should do root first and then home. Any advice or help is appreciated!

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[–] colin@lemmy.uninsane.org 1 points 2 years ago (1 children)

i migrated from a stock nixos deployment to an amnesiac one, without creating a totally new filesystem/install (i.e. without “deleting my entire root”). right now you have a fs mounted to /. after impermanence, you’ll mount that fs to /nix and bind-mount things from /nix/persist back to parts of the rootfs. if you can access the fs offline, then you can move /nix/* up to the root of that fs, mount it at /nix instead of / during boot, and now your system’s amnesiac. all your old state will be in /nix/… instead of where anything might expect it. move that to /nix/persist, and then start adding impermanence entries for the state you want to persist.

you can actually do this online if you’re careful (cp /nix/* up to /, then edit the fileSystems entry so that your media is mounted at /nix, nixos-rebuild switch, cp again to make sure the new generation is in both places, reboot, delete the old store which should be visible at /nix/nix/store, then organize the persisted data as above).

safety tips are:

don’t delete any files until you’re sure they’re not needed.
if you’re okay working offline, then prefer mving stuff into where it’s supposed to be and just undoing that if it doesn’t boot.
if you have to work online, then don’t move anything but copy it instead, and you’ll always be able to reboot into older, known-good generations (assuming /boot is on its own fs).
take notes about where you mv stuff so that you know how to undo things if you go wrong.
join us in the #impermanence:nixos.org Matrix channel if you’re not already there :)

[–] ruination@discuss.tchncs.de 1 points 2 years ago (1 children)

Oh nice, I didn't realise there's an entire Matrix channel for that, I'll join in a bit then. I'm assuming by "working offline" you mean booting into a live USB? Also, is that what the Impermanence repository does on the nix-community?

[–] colin@lemmy.uninsane.org 1 points 2 years ago (1 children)

I’m assuming by “working offline” you mean booting into a live USB

yeah, that's one way. more generally, "offline" = any setup where the filesystem(s) you're modifying to be impermanent isn't critical to running whatever shell/session you're using to do the work. that lets you do things in any order so long as the end state's bootable, whereas moving /nix/store on a live system can totally just crash it and leave it unbootable if you're not careful.

Also, is that what the Impermanence repository does on the nix-community?

default behavior for that impermanence repo is that persisted directories are mounted as bind mounts and persisted files are symlinks. that was chosen mostly to maximize out-of-the-box compatibility with most software out there, i believe. most people mount some fs to /nix during early boot, and place their persisted data in the persist/ subdirectory of that mount. but there's a hundred different ways to do it: the other most common way is to use subvolumes and snapshots (btrfs, but maybe zfs can do it too?): / is one subvolume, and you take a snapshot where it's empty; /nix is a subvolume where everything gets persisted. the Impermanence config that mounts stuff from /nix/persist into where you want it looks the same as it does with a tmpfs-on-root setup, and you get impermanence by just reverting / to that original snapshot on every boot.

[–] ruination@discuss.tchncs.de 1 points 2 years ago* (last edited 2 years ago) (1 children)

Yeah, ZFS can do it too, in fact I think that's what Graham used in "Erase your darlings". I'm thinking of just going the tmpfs root route, making a new dataset for /persist, and copying over the necessary files from my original root dataset, and using the impermanence module to take care of the rest of the work. That way, if I miss some files, I can just go back and copy them over from the root dataset to the persist dataset, and once I'm sure I got everything, I can just destroy the old root dataset. Would that be a good way to go about it?

[–] colin@lemmy.uninsane.org 1 points 2 years ago (1 children)

sounds like one of the more battle-tested routes, yeah 👍️

[–] ruination@discuss.tchncs.de 2 points 2 years ago

Alright, thanks for all your help! I'll give it a shot tomorrow.

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