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
614785886,MDU6SXNzdWU2MTQ3ODU4ODY=,4046,automatic chunking of zarr archive,11750960,open,0,,,3,2020-05-08T14:42:00Z,2023-01-18T21:54:42Z,,CONTRIBUTOR,,,,"I store data in a zarr archive that is not chunked and the resulting zarr archive is chunked.
This may be as simple usage question.
I don't know how to turn this behavior off.

#### Code sample

Here is minimal example that reproduces the issue:

```python
ds = xr.DataArray(np.ones((200,800))).rename('foo').to_dataset()
print('Initial chunks = {}'.format(ds.foo.chunks))
ds.to_zarr('test.zarr', mode='w')
print('zarr archives contains: {}'.format(os.listdir('test.zarr/foo')))
ds = xr.open_zarr('test.zarr')
print('Final chunks = {}'.format(ds.foo.chunks))
```
returns:
```
Initial chunks = None
zarr archives contains: ['.zarray', '.zattrs', '0.0', '0.1', '1.0', '1.1']
Final chunks = ((100, 100), (400, 400))
```

#### Expected Output

I would expect the archive to not to be chunked.

#### Versions

<details><summary>Output of <tt>xr.show_versions()</tt></summary>


INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 | packaged by conda-forge | (default, Mar 23 2020, 23:03:20) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 3.12.53-60.30-default
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.4

xarray: 0.15.2.dev29+g6048356
pandas: 1.0.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.4.0
cftime: 1.1.1.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.13.0
distributed: 2.13.0
matplotlib: 3.2.1
cartopy: 0.17.0
seaborn: 0.10.0
numbagg: None
pint: None
setuptools: 46.1.3.post20200325
pip: 20.0.2
conda: None
pytest: None
IPython: 7.13.0
sphinx: None

</details>
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283518232,MDU6SXNzdWUyODM1MTgyMzI=,1795,open_mfdataset concat_dim chunk,11750960,open,0,,,2,2017-12-20T10:34:58Z,2020-01-07T16:19:39Z,,CONTRIBUTOR,,,,"open_mfdataset does not allow chunking along concat_dim.

As a result if specific chunking is sought along that dimension by the user it may be best not to pass chunks at the open_mfdataset stage and rechunk variables afterwards.
This would be the case for example if chunks are large across files but small within files:
https://github.com/apatlpo/lops-array/blob/master/sandbox/natl60_tseries_debug.ipynb

I believe this is difficult to anticipate for new users (like me).

Couldn't this be specified in the documentation of open_mfdataset?

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