Zarr chunking (GH2300)

doc/whats-new.rst

@@ -136,6 +136,10 @@ Bug fixes
   the dates must be encoded using cftime rather than NumPy (:issue:`2272`).
   By `Spencer Clark <https://github.com/spencerkclark>`_.
 
+- Chunked datasets can now roundtrip to Zarr storage continually 
+  with `to_zarr` and ``open_zarr`` (:issue:`2300`).
+  By `Lily Wang <https://github.com/lilyminium>`_.
+
 .. _whats-new.0.10.9:
 
 v0.10.9 (21 September 2018)

xarray/backends/zarr.py

@@ -79,14 +79,14 @@ def _determine_zarr_chunks(enc_chunks, var_chunks, ndim):
     if var_chunks and enc_chunks is None:
         if any(len(set(chunks[:-1])) > 1 for chunks in var_chunks):
             raise ValueError(
-                "Zarr requires uniform chunk sizes excpet for final chunk."
-                " Variable %r has incompatible chunks. Consider "
+                "Zarr requires uniform chunk sizes except for final chunk."
+                " Variable dask chunks %r are incompatible. Consider "
                 "rechunking using `chunk()`." % (var_chunks,))
         if any((chunks[0] < chunks[-1]) for chunks in var_chunks):
             raise ValueError(
-                "Final chunk of Zarr array must be smaller than first. "
-                "Variable %r has incompatible chunks. Consider rechunking "
-                "using `chunk()`." % var_chunks)
+                "Final chunk of Zarr array must be the same size or smaller "
+                "than the first. Variable Dask chunks %r are incompatible. "
+                "Consider rechunking using `chunk()`." % var_chunks)
         # return the first chunk for each dimension
         return tuple(chunk[0] for chunk in var_chunks)
 
@@ -126,14 +126,21 @@ def _determine_zarr_chunks(enc_chunks, var_chunks, ndim):
     # threads
     if var_chunks and enc_chunks_tuple:
         for zchunk, dchunks in zip(enc_chunks_tuple, var_chunks):
-            for dchunk in dchunks:
+            for dchunk in dchunks[:-1]:
                 if dchunk % zchunk:
                     raise NotImplementedError(
                         "Specified zarr chunks %r would overlap multiple dask "
                         "chunks %r. This is not implemented in xarray yet. "
                         " Consider rechunking the data using "
                         "`chunk()` or specifying different chunks in encoding."
                         % (enc_chunks_tuple, var_chunks))
+            if dchunks[-1] > zchunk:
+                raise ValueError(
+                    "Final chunk of Zarr array must be the same size or "
+                    "smaller than the first. The specified Zarr chunk "
+                    "encoding is %r, but %r in variable Dask chunks %r is "
+                    "incompatible. Consider rechunking using `chunk()`."
+                    % (enc_chunks_tuple, dchunks, var_chunks))
         return enc_chunks_tuple
 
     raise AssertionError(

xarray/tests/test_backends.py

@@ -1389,6 +1389,10 @@ def test_chunk_encoding_with_dask(self):
         ds_chunk_irreg = ds.chunk({'x': (5, 5, 2)})
         with self.roundtrip(ds_chunk_irreg) as actual:
             assert (5,) == actual['var1'].encoding['chunks']
+        # re-save Zarr arrays
+        with self.roundtrip(ds_chunk_irreg) as original:
+            with self.roundtrip(original) as actual:
+                assert_identical(original, actual)
 
         # - encoding specified  -
         # specify compatible encodings