I'm not really sure what to think any more — we have had a real, consistent issue that seemed to fit the description of this issue which went away with one of the fixes above (using single threading), but using local files at the moment seems to remove the error even with the current version of xarray and either parallel option.

Ok I downloaded the two files and indeed there is no error with parallel=True nor parallel=False.

@jhamman Yes, using the PR version of xarray, with parallel=True I met the error but with parallel=False I did not.

I was able to reproduce the error with the current version of xarray and then have it work with the new version. Here is what I did:

Make new environment conda create -n test_xarray xarray netcdf4 dask

Check version ``` (test_xarray) kthyng@adams ~ % conda list xarray

packages in environment at /Users/kthyng/miniconda3/envs/test_xarray:

Name Version Build Channel

xarray 2023.3.0 pyhd8ed1ab_0 conda-forge ```

In python: import xarray as xr urls = ["https://opendap.co-ops.nos.noaa.gov/thredds/dodsC/NOAA/WCOFS/MODELS/2023/03/31/nos.wcofs.2ds.n001.20230331.t03z.nc", "https://opendap.co-ops.nos.noaa.gov/thredds/dodsC/NOAA/WCOFS/MODELS/2023/03/31/nos.wcofs.2ds.n002.20230331.t03z.nc"] xr.open_mfdataset(urls) returns the following the first time xr.open_mfdataset(urls) is run but the second time it runs fine. OSError: [Errno -70] NetCDF: DAP server error: 'https://opendap.co-ops.nos.noaa.gov/thredds/dodsC/NOAA/WCOFS/MODELS/2023/03/31/nos.wcofs.2ds.n002.20230331.t03z.nc'

Next I used the PR version of xarray and reran the code above and then it was able to read in ok on the first try.

Note: after a week or so those files won't work and will have to be updated with something more current but the pattern to use is clear from the file names.

@jhamman Sorry for my delay — I started this the other day and got waylaid. I'll try to get back to it today or tomorrow.

iris has the pin in their package metadata

Note that this will hopefully be removed soon - SciTools/iris#5095 - but the reviewer has been assigned to other urgent work so it's paused right now.

@ocefpaf and all: thank you! What a mysterious error this has been. Using the workaround

import dask dask.config.set(scheduler="single-threaded")

did indeed avoid the issue for me.

I ran into this problem yesterday reading netcdf files on our HPC with a known good script and netcdf files. Unfortunately just trying to open the files again in a try..except block did not work for me. Looking back through my environment update history I found that the netcdf4 library had been updated since I'd last successfully run the script. The current version installed was conda-forge/linux-64::netcdf4-1.6.1-nompi_py39hfaa66c4_100; I rolled it back to conda-forge/linux-64::netcdf4-1.6.0-nompi_py39h6ced12a_102. After the rollback the script started working again without error.