Nan values when saving parq files with virtualize.to_kerchunk()

[QuentinMaz]

Hi,

I have used virtualizarr to concatenate several .nc files into one parq one.
I noticed that when I then open the saved dataset, the first value of its index is replaced with nan.
I thus suspect that virtualize.to_kerchunk() might have a bug.

Here how to replicate the issue:

filename = "test"
# synthetic xarray.DataSet inspired by xarray's documentation
temperature = 15 + 8 * np.random.randn(2, 2, 3)
lon = [[-99.83, -99.32], [-99.79, -99.23]]
lat = [[42.25, 42.21], [42.63, 42.59]]
depths = np.arange(150, step=50)
da = xr.DataArray(
    data=temperature,
    dims=["x", "y", "depth"],
    coords=dict(
        lon=(["x", "y"], lon),
        lat=(["x", "y"], lat),
        depth=depths
    ),
    attrs=dict(
        description="Ambient temperature.",
        units="degC",
    ),
)
ds = da.to_dataset(name="temperature")
ds.to_netcdf(f"{filename}.nc")
vds = open_virtual_dataset(
    f"{filename}.nc",
    indexes={}, 
    decode_times=True, 
    loadable_variables=["lon", "lat", "depth"]
)
print("depth index of vds:\t\t", vds.depth.to_numpy())
# depth index of vds:		 [  0  50 100]

# saves as parq/ folder
vds.virtualize.to_kerchunk(f"{filename}.parq", format="parquet")
loaded_ds = xr.open_dataset(f"{filename}.parq", engine="kerchunk", chunks={})
print("depth index of the loaded vds:\t", loaded_ds.depth.to_numpy())
# depth index of the loaded vds:	 [ nan  50. 100.]

# temporary fix
loaded_ds.coords["depth"].values[0] = 0.
print("index after fix:\t\t", loaded_ds.depth.to_numpy())
# index after fix:		 [  0.  50. 100.]

I am a beginner and have therefore no idea of the cause...

[norlandrhagen]

Hey there @QuentinMaz! Thanks for trying out VirtualiZarr and opening up a clear MRE.

Definitely seems like an issue. On some initial digging it seems like:

• VirtualiZarr and Kerchunk's SingleHDF5ToZarr both point to the same on-disk reference for depth:

# Virtualizarr Depth manifest 
{'0': {'path': 'file:///<...>/test.nc','offset': 8256, 'length': 24}}
# kerchunk Depth reference
#   'depth/0': ['test.nc', 8256, 24],

• Writing to .json or .parquet produces the same missing first value in depth.
• It seems like when using Kerchunk to write the reference .json, the fill value for depth is null. When writing with .virtualize.to_kerchunk(), then fill_value for depth is 0. It seems like we're getting a fill_value coercion from None to 0 based on the float dtype :

  VirtualiZarr/virtualizarr/zarr.py

  Line 64 in af9c374

  if self.fill_value is None:

I'll try to dig into this further, in the meantime if you're open to working a bit on the bleeding edge, you could try writing the references to Icechunk. It might take a bit of environment-fu since Kerchunk doesn't yet support Zarr V3. To keep a single environment, you could use the new Zarr-V3 compliant hdf5 reader, then write to Icechunk.

from virtualizarr.readers.hdf import HDFVirtualBackend

vds = open_virtual_dataset('file.nc', backend=HDFVirtualBackend)

[QuentinMaz]

Thanks for the comments and answer @norlandrhagen!
I suspected indeed that the nan value might be due to the initial value being 0 especially.
I am a kind of beginner on the project I have recently joined, so I think I will stick to my temporary fix ;) But for sure I will try later your suggestions!

Even though I am pretty sure to not be skilful enough to help through your investigations, I will have a look at the status of the issue to (try to) follow your progress :) Good luck!