Hi all,

We are trying to specify chunks after opening an mfdataset but it does not work. This works fine with datasets. Is this behavior expected? Are we doing anything wrong?

```

- Modules

import sys, os import xarray as xr

chunks = (1727, 2711) xr_chunks = {'x': chunks[-1], 'y': chunks[-2], 'time_counter':1, 'deptht': 1}

- Parameter

natl60_path = '/home7/pharos/othr/NATL60/' filename = natl60_path+'NATL60-MJM155-S/5d/2008/NATL60-MJM155_y2008m01d09.5d_gridT.nc' filenames = natl60_path+'NATL60-MJM155-S/5d/2008/NATL60-MJM155_y2008m01d0*gridT.nc'

dataset

open

ds = xr.open_dataset(filename,chunks=None)

chunk

ds = ds.chunk(xr_chunks)

plot

print 'With dataset:' print ds['votemper'].isel(time_counter=0,deptht=0).values

mfdataset

open

ds = xr.open_mfdataset(filenames,chunks=None, lock=False)

plot

print 'With mfdataset no chunks:' print ds['votemper'].isel(time_counter=0,deptht=0).values

chunk

print 'With mfdataset with chunks:' ds = ds.chunk(xr_chunks) print ds['votemper'].isel(time_counter=0,deptht=0) print ds['votemper'].isel(time_counter=0,deptht=0).values ```

The output is:

With dataset: [[ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan] ..., [ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan]] With mfdataset no chunks: [[ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan] ..., [ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan] [ nan nan nan ..., nan nan nan]] With mfdataset with chunks: <xarray.DataArray 'votemper' (y: 3454, x: 5422)> dask.array<getitem..., shape=(3454, 5422), dtype=float64, chunksize=(1727, 2711)> Coordinates: nav_lat (y, x) float32 26.5648 26.5648 26.5648 26.5648 26.5648 ... nav_lon (y, x) float32 -81.4512 -81.4346 -81.4179 -81.4012 ... deptht float32 0.480455 * x (x) int64 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 ... * y (y) int64 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 ... time_counter datetime64[ns] 2008-01-02T12:00:00 time_centered datetime64[ns] 2008-01-02T12:00:00 Attributes: long_name: temperature units: degC online_operation: average interval_operation: 40s interval_write: 5d

The code hangs for a while and then spits:

``` Traceback (most recent call last): File "/home/slyne/aponte/natl60/python/natl60_dimup/overview/aurelien/plot_snapshot_2d_v4_break.py", line 44, in <module> print ds['votemper'].isel(time_counter=0,deptht=0).values File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/xarray/core/dataarray.py", line 364, in values return self.variable.values File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/xarray/core/variable.py", line 288, in values return _as_array_or_item(self._data_cached()) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/xarray/core/variable.py", line 254, in _data_cached self._data = np.asarray(self._data) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/numpy/core/numeric.py", line 460, in asarray return array(a, dtype, copy=False, order=order) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/array/core.py", line 867, in array x = self.compute() File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/base.py", line 37, in compute return compute(self, kwargs)[0] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/base.py", line 110, in compute results = get(dsk, keys, kwargs) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/threaded.py", line 57, in get **kwargs) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 481, in get_async raise(remote_exception(res, tb)) dask.async.MemoryError:

Traceback

File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 264, in execute_task result = _execute_task(task, data) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 245, in _execute_task args2 = [_execute_task(a, cache) for a in args] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 245, in _execute_task args2 = [_execute_task(a, cache) for a in args] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 242, in _execute_task return [_execute_task(a, cache) for a in arg] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 242, in _execute_task return [_execute_task(a, cache) for a in arg] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 242, in _execute_task return [_execute_task(a, cache) for a in arg] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 242, in _execute_task return [_execute_task(a, cache) for a in arg] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 245, in _execute_task args2 = [_execute_task(a, cache) for a in args] File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/async.py", line 246, in _execute_task return func(*args2) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/dask/array/core.py", line 50, in getarray c = np.asarray(c) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/numpy/core/numeric.py", line 460, in asarray return array(a, dtype, copy=False, order=order) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/xarray/core/indexing.py", line 312, in array return np.asarray(array[self.key], dtype=None) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/xarray/conventions.py", line 359, in getitem self.scale_factor, self.add_offset, self._dtype) File "/home1/homedir5/perso/aponte/miniconda2/envs/natl60/lib/python2.7/site-packages/xarray/conventions.py", line 57, in mask_and_scale values = np.array(array, dtype=dtype, copy=True) ```

Cheers

Aurelien