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Comment by jbott | Hacker News Reader

This is linked in the footer of this page, but Graham Christensen has an excellent blog post “Erase your darlings” [1] that explains why you might want to do this. There is a script floating around on github [2] that does the install automatically, but I needs to be modified slightly to work on newer nixos versions. I have a forked version [3] that I used this morning, but my decisions might not make sense for everyone, but it’s provided as-is for now :)

1. https://grahamc.com/blog/erase-your-darlings/

2. https://gist.github.com/mx00s/ea2462a3fe6fdaa65692fe7ee824de...

3. https://gist.github.com/jbott/531b9d555dae7f197f25326ef251f1...

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There’s an even better setup than Erase your Darlings. Instead of putting / on a zfs pool and erasing and rewriting it every reboot, just put tmpfs on / instead.

With / all in memory it automatically gets wiped and recreated every reboot, without needing to actively erase a disk, and thus with much less drive wear (depending on how frequently you reboot). It’s also faster on some things when loading from RAM instead of the disk. And it’s overall a cleaner, simpler setup.

https://elis.nu/blog/2020/05/nixos-tmpfs-as-root/

You can also put tmpfs on home as well, using Impermanence and Home Manager to persist things like ~/.config and whatever other files or folders need to persist between reboots:

https://elis.nu/blog/2020/06/nixos-tmpfs-as-home/

tmpfs on / is enabled by NixOS’s unique design, in which it keeps the entire system in /nix/store and then softlinks all the paths into their appropriate place in /. With tmpfs on /, NixOS automatically recreates those softlinks in tmpfs on reboot. Very little setup effort is required to make this work.

I don’t think the point about disk wear is true. ZFS is a block-based CoW filesystem already, so the “erase” is actually something more like “make a new metadata entry that points to an earlier state snapshot”. Plus, I’d expect normal disk usage (ie. chrome disk cache) probably far outweighs any disk wear you’d get from the (relatively) small size of files you get written into /.

Unless you’re working on a server with a ton of ram, I also think using tmpfs is more likely to shoot yourself in the foot with excess memory pressure. I don’t know of a way for the kernel to free memory if you write a huge file to the tmpfs partition by mistake, unless you use swap, and then you have the problems that come with that.

Tmpfs might be faster though!

> tmpfs is more likely to shoot yourself in the foot with excess memory pressure

It defaults to 50% of physical RAM, but you can set the size option to whatever you want: https://www.kernel.org/doc/html/latest/filesystems/tmpfs.htm...

(So yes, something to consider, but totally possible to mitigate)

TIL!

You do need enough RAM for this, though RAM is relatively cheap these days.

I was aware of the possibility of just using tmpfs, but I went with ZFS for / anyway for mostly one reason: I can erase root on every boot, but still keep snapshots of the last few boots. That means, that if I mess up some configuration and I fail to persist some important data, I can still go back and recover it if I need to.

I'm not sure that's necessary for / at least, though /home is a different matter.

Everything that needs to be persisted for / is done so either in configuration.nix (or equivalent flake), or using Impermanence. Once you've persisted things in one of those, it will remain so for all future derivations.

You can also create separate ZFS pools for things like LXC/LXD that need to persist between boots, but don't naturally go in / or /home.

So far I haven't run into a case where I forgot to add something to my impermanence config, so you may be right: it might not be necessary. However, just knowing that I could fix it if I forget something (for example when setting up a new service) somewhere down the line just gives me peace of mind.

That's exactly what this HN post is about. The wiki page being linked to has instructions for doing that.

You can also use have tmpfs on / as an overlay file system. This is quite common for network boot devices that all use a shared nfs root.

Sounds interesting in theory, but modern development setups are often full with tools, dependencies, IDE indices, intermediate build results, etc. pp. that all may take a very long time to download and build from scratch. Sure, you could try to keep track of all such things and persist them but it's going to be a lot of effort trying to figure out where all your tools are dumping their state.

it's not so difficult, really. most of this is in your home folder, which is just a separate persistent mount.

you can also use impermanence with home-manager if you want

https://github.com/nix-community/impermanence#home-manager

Sure, but then you're throwing away the purported benefits (setup is always reproducible) for your entire home folder.

It seems like there are two contradictory goals here, each with their own benefits. Impermanence gives you reproducibility, but permanence gives you performance.

Maybe the right tradeoff is to just persist the entire home folder and nothing else (or few other things... I'm not sure I'd want to always download some programs that are quite huge), but the tradeoff is essentially still there.

you're talking about three different things: NixOS, Home Manager, and Impermanence. or put another way: reproducible root filesystem, reproducible home folder, automatically reproducing things on boot.

if you want a reproducible root filesystem, use NixOS. if you want to reproduce your root filesystem on boot, use NixOS with impermanence.

if you want a reproducible home folder, use home manager. if you want to reproduce your home folder on boot, use home manager with impermanence.

I only use impermanence on servers. I don't think there's any point using it on a desktop, or with Home Manager, other than street cred. it just complicates things. I don't really even recommend Home Manager for first time NixOS users for the same reason.

edit: and I want to address this comment:

> I'm not sure I'd want to always download some programs that are quite huge

this isn't how it works; the binaries, as well as the entire initial state of the filesystem, are cached in the Nix cache, a read-only filesystem. you're not downloading everything on every boot.

> often full with tools, dependencies, IDE indices, intermediate build results

Tools and dependencies go in nix. Indexes and temporary build stuff is just cache, which you can still have (I'd lean towards regenerating per reboot, but YMMV).

> but it's going to be a lot of effort trying to figure out where all your tools are dumping their state.

Fair. Kind of an indictment of the current state of the ecosystem, but yes.

I don't think of impermanence as a tool for development setups, but rather a tool to improve production security. When a server gets compromised, it's common for an attacker to leverage their initial access to set up backdoor access for themselves, e.g. an additional privileged user or privileged service which phones home, so that they're no longer reliant on the original vulnerability to gain access again. This is important to ensure that they can launch a more damaging attack at a more opportune time (e.g. at the beginning of a long weekend). Now consider a stateful server which you need to host (e.g. Kubernetes control plane / etcd) where you ordinarily cannot practice immutable infrastructure due to the stateful nature of the server. Modules like impermanence allow you to guard against this kind of compromise by simply wiping out everything but the actual state as a result of rebooting. Any privileged users or malicious processes (which, of course, are not part of the system configuration used at boot) get wiped out at every reboot. It's not a silver bullet - an attacker could simply releverage the original vulnerability and set up access again - but doing the reboots frequently would force the vulnerability to be re-exploited each time, making it a pattern of access more likely to be detected in a SIEM.

With NixOS, you typically use flake.nix or shell.nix to "shell into" development environment so this is kinda non issue. You can also use `nix run` or `nix-env` to mix traditional package management and this.

> but it's going to be a lot of effort trying to figure out where all your tools are dumping their state.

This is true though, if you dont want to manage all of their configs from nix, I would make their dump locations, or $XDG_DIRS/$HOME (if you are lazy) permanent in this case.

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