Storage optimization: Difference between revisions

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=== Pinning ===

~~When you invoke <code>nix-shell</code> with~~:

Running the following command:

$ nix-instantiate shell.nix --indirect --add-root ~~$DIR~~/.nix-gc-roots/shell.drv ...

$ nix-instantiate shell.nix --indirect --add-root ./.nix-gc-roots/shell.drv ...

</syntaxhighlight>

~~Then you'll have~~ a persistent ~~environment~~ which won't be garbage collected as long as you have configured <code>keep-outputs = true</code>. This is useful ~~when~~ you don't want to spend time waiting ~~for redownloads~~ every time you enter the shell.

Will create a persistent snapshot of your <code>shell.nix</code> dependencies, which then won't be garbage collected, as long as you have configured <code>keep-outputs = true</code> (and haven't changed the default of <code>keep-derivations = true</code>). This is useful if your project has a dependency with no substitutes available, or you don't want to spend time waiting to re-download your dependencies every time you enter the shell.

~~A little problem exists though. GC roots are numbered sequentially, so if you change <code>shell.nix</code>~~ to ~~contain '''fewer''' derivations, such~~ that ~~the name of the last GC root starts with~~ <code>~~shell.drv~~-7</code>~~, then~~ <code>shell.~~drv-{8,9,10,11,12}*</code> will be dangling and unused. To overcome this problem you should remove GC roots dir periodically (or just before running <code>~~nix~~-shell~~</code>).

You need to re-run that <code>nix-instantiate</code> command any time your <code>shell.nix</code> changes.

~~Obviously~~, you ~~should remove~~ the GC roots directory ~~for projects~~ you ~~don~~'t ~~plan~~ to ~~work on~~.

And there is a subtle gotcha if your <code>shell.nix</code> happens to evaluate to more than one derivation: <code>nix-instantiate</code> will number each derivation sequentially, so if you change your <code>shell.nix</code> to contain ''fewer'' derivations, such that (for example) the name of the last GC root starts with <code>shell.drv-7</code>, then <code>shell.drv-{8,9,10,11,12...}</code> will be dangling and unused.

The easiest way to get around this is to delete the <code>./.nix-gc-roots</code> directory periodically (i.e., any time you re-run the <code>nix-instantiate</code> command).

Don't forget to periodically check your GC roots, and remove any that you no longer need.

=== Automation ===

@@ Line 94: / Line 94: @@
 === Pinning ===
-When you invoke <code>nix-shell</code> with:
+Running the following command:
 <syntaxhighlight lang="console">
-$ nix-instantiate shell.nix --indirect --add-root $DIR/.nix-gc-roots/shell.drv ...
+$ nix-instantiate shell.nix --indirect --add-root ./.nix-gc-roots/shell.drv ...
 </syntaxhighlight>
-Then you'll have a persistent environment which won't be garbage collected as long as you have configured <code>keep-outputs = true</code>. This is useful when you don't want to spend time waiting for redownloads every time you enter the shell.
+Will create a persistent snapshot of your <code>shell.nix</code> dependencies, which then won't be garbage collected, as long as you have configured <code>keep-outputs = true</code> (and haven't changed the default of <code>keep-derivations = true</code>). This is useful if your project has a dependency with no substitutes available, or you don't want to spend time waiting to re-download your dependencies every time you enter the shell.
-A little problem exists though. GC roots are numbered sequentially, so if you change <code>shell.nix</code> to contain '''fewer''' derivations, such that the name of the last GC root starts with <code>shell.drv-7</code>, then <code>shell.drv-{8,9,10,11,12}*</code> will be dangling and unused. To overcome this problem you should remove GC roots dir periodically (or just before running <code>nix-shell</code>).
+You need to re-run that <code>nix-instantiate</code> command any time your <code>shell.nix</code> changes.
-Obviously, you should remove the GC roots directory for projects you don't plan to work on.
+And there is a subtle gotcha if your <code>shell.nix</code> happens to evaluate to more than one derivation: <code>nix-instantiate</code> will number each derivation sequentially, so if you change your <code>shell.nix</code> to contain ''fewer'' derivations, such that (for example) the name of the last GC root starts with <code>shell.drv-7</code>, then <code>shell.drv-{8,9,10,11,12...}</code> will be dangling and unused.
+The easiest way to get around this is to delete the <code>./.nix-gc-roots</code> directory periodically (i.e., any time you re-run the <code>nix-instantiate</code> command).
+Don't forget to periodically check your GC roots, and remove any that you no longer need.
 === Automation ===