id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
874331538,MDExOlB1bGxSZXF1ZXN0NjI4OTE0NDQz,5252,"Add mode=""r+"" for to_zarr and use consolidated writes/reads by default",1217238,closed,0,,,14,2021-05-03T07:57:16Z,2021-06-22T06:51:35Z,2021-06-17T17:19:26Z,MEMBER,,0,pydata/xarray/pulls/5252,"`mode=""r+""` only allows for modifying pre-existing array values in a
Zarr store. This makes it a safer default `mode` when doing a limited
`region` write. It also offers a nice performance bonus when using
consolidated metadata, because the store to modify can be opened in
""consolidated"" mode -- rather than painfully slow non-consolidated
mode.

This PR includes several related changes to `to_zarr()`:

1. It adds support for the new `mode=""r+""`.
2. `consolidated=True` in `to_zarr()` now means ""open in consolidated
   mode"" if using using `mode=""r+""`, instead of ""write in consolidated
   mode"" (which would not make sense for r+).
3. It allows setting `consolidated=True` when using `region`, mostly
   for the sake of fast store opening with r+.
4. Validation in `to_zarr()` has been reorganized to always use the
   _existing_ Zarr group, rather than re-opening zar stores from
   scratch, which could require additional network requests.
5. Incidentally, I've renamed the `ZarrStore.ds` attribute to
   `ZarrStore.zarr_group`, which is a much more descriptive name.

These changes gave me a ~5x boost in write performance in a large
parallel job making use of `to_zarr` with `region`.

<!-- Feel free to remove check-list items aren't relevant to your change -->

- [x] Tests added
- [x] Passes `pre-commit run --all-files`
- [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`
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