dvc exp save target

[gregstarr]

How come we can dvc exp run <target> but we can't dvc exp save <target>?

[pmrowla]

Experiment commits capture the state of your entire repository at the time of the commit (and not just the state of a specific target).

So for dvc exp save specifying a target wouldn't actually have any effect (it just captures the repository state at the time you run the command). For dvc exp run, you can specify a target because DVC reproduces those stage(s) before capturing the repository state.

[gregstarr]

Do dvc exp run and dvc exp save have different behavior when it comes to saving though? dvc exp save complains if things are missing e.g.

ERROR: failed to save experiment - output 'pipelines/big/models/surface.keras' does not exist 

but dvc exp run <target> doesn't have any problem with this.

[pmrowla]

This is probably an exp save bug, it should behave the same as exp run in this situation (and just save whatever is available). Feel free to open a separate issue for this

[dberenbaum]

Does dvc exp run generate that pipelines/big/models/surface.keras output? I think both will fail if that does not exist by the end of the experiment.

[gregstarr]

no exp run will not generate that output. exp run will only sort-of fail. It will still reproduce the outputs I want and save them. The experiment will be red in the vscode extension and complain about something failing but it will still basically work.

[dberenbaum]

Got it. So if you have two pipelines, you can run one and save the results with dvc exp run <target> even if the other is missing outputs.

@pmrowla I think this would also fail in dvc exp run without a target. Consider this dvc.yaml:

stages:
  one:
    cmd: echo one > one.txt
    outs:
      - one.txt
  two:
    cmd: echo two > /dev/null
    outs:
      - two.txt

dvc exp run fails with the error ERROR: failed to reproduce 'two': output 'two.txt' does not exist. I think it makes sense to support dvc exp save <target>, which will narrow the dvc commit part of the operation (even though the git commit is for the entire repo).