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https://github.com/pydata/xarray/issues/2329#issuecomment-409172635 https://api.github.com/repos/pydata/xarray/issues/2329 409172635 MDEyOklzc3VlQ29tbWVudDQwOTE3MjYzNQ== 10050469 2018-07-31T10:25:16Z 2018-07-31T14:18:29Z MEMBER

Sorry for the confusion, I had an obvious mistake in my timing experiment above (forgot to do the actual computations...).

The dimension order does make a difference:

```python import dask as da import xarray as xr

d = xr.DataArray(da.array.zeros((1000, 721, 1440), chunks=(10, 721, 1440)), dims=('z', 'y', 'x')) d.to_netcdf('da.nc') # 8.3 Gb

with xr.open_dataarray('da.nc', chunks={'z':10}) as d: %timeit d.sum().load() 3.94 s ± 95.5 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

with xr.open_dataarray('da.nc', chunks={'y':10}) as d: %timeit d.sum().load() 4.15 s ± 316 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

with xr.open_dataarray('da.nc', chunks={'x':10}) as d: %timeit d.sum().load() 1min 54s ± 1.43 s per loop (mean ± std. dev. of 7 runs, 1 loop each)

with xr.open_dataarray('da.nc', chunks={'y':10, 'x':10}) as d: %timeit d.sum().load() 2min 23s ± 215 ms per loop (mean ± std. dev. of 7 runs, 1 loop each) ```

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