Keep last "known good" profile when running nix-collect-garbage

[l0b0]

Is your feature request related to a problem? Please describe.

Running with the --delete-old flag makes any rollback impossible, which is risky. And picking a reasonable --delete-older-than period to keep at least N profiles while reducing disk usage as much as possible means having to look through the list of generations to work out the amount of time since the last N builds and running with that plus a bit of a margin. This is cumbersome.

Describe the solution you'd like
The ideal solution would keep only the last known good profile plus the current profile. That way a working fallback is always available, and I can still attempt booting into the new (untested) profile, while ensuring minimal disk use.

This might be complicated to achieve, especially since "known good" is hard to pin down even for system profiles. For example, a system might boot, and might even start an X server, only to consistently crash a few seconds later due to a video driver bug (this happened to me recently).

A good enough solution might be a flag to keep the last N profiles.

How this interacts with --delete-old and --delete-older-than is probably not that important.

[grahamc]

One option could make making a separate profile called “known-good” and updating that profile when you know a generation is good.

On December 22, 2021, GitHub ***@***.***> wrote: Is your feature request related to a problem? Please describe. Running with the --delete-old flag makes any rollback impossible, which is risky. And picking a reasonable --delete-older-than period to keep at least N profiles while reducing disk usage as much as possible means having to look through the list of generations to work out the amount of time since the last N builds and running with that plus a bit of a margin. This is cumbersome. Describe the solution you'd like The ideal solution would keep only the last known good profile plus the current profile. That way a working fallback is always available, and I can still attempt booting into the new (untested) profile, while ensuring minimal disk use. This might be complicated to achieve, especially since "known good" is hard to pin down even for system profiles. For example, a system might boot, and might even start an X server, only to consistently crash a few seconds later due to a video driver bug (this happened to me recently). A good enough solution might be a flag to keep the last N profiles. How this interacts with --delete-old and --delete-older-than is probably not that important.

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[vcunat]

[...] look through the list of generations to work out the amount of time since the last N builds and running with that plus a bit of a margin. This is cumbersome.

Workaround: if you go to such lengths, I'd rather just "manually" delete the symlinks according to your wishes. (And it's quite easy to script one-liners for last-N, etc.)

[stale]

I marked this as stale due to inactivity. → More info

[cnnrro]

A good enough solution might be a flag to keep the last N profiles.

Another workaround for this particular solution:

You can garbage collect all but the last N system profiles by running: nix-env --profile /nix/var/nix/profiles/system --delete-generations +N ¹ followed by the usual nix-collect-garbage.

A simple maintenance script might look something like:

nix-env --profile /nix/var/nix/profiles/system --delete-generations +5
nix-collect-garbage
nix-store --optimise

Footnotes

1. https://nixos.org/manual/nix/stable/command-ref/nix-env.html#operation---delete-generations ↩