An update on this long-standing issue.

I have learned that open_mfdataset can be blazingly fast if decode_cf=False but extremely slow with decode_cf=True.

As an example, I am loading a POP datataset on cheyenne. Anyone with access can try this example. ```python base_dir = '/glade/scratch/rpa/' prefix = 'BRCP85C5CN_ne120_t12_pop62.c13b17.asdphys.001' code = 'pop.h.nday1.SST' glob_pattern = os.path.join(base_dir, prefix, '%s.%s.*.nc' % (prefix, code))

def non_time_coords(ds): return [v for v in ds.data_vars if 'time' not in ds[v].dims]

def drop_non_essential_vars_pop(ds): return ds.drop(non_time_coords(ds))

this runs almost instantly

ds = xr.open_mfdataset(glob_pattern, decode_times=False, chunks={'time': 1}, preprocess=drop_non_essential_vars_pop, decode_cf=False) And returns this <xarray.Dataset> Dimensions: (d2: 2, nlat: 2400, nlon: 3600, time: 16401, z_t: 62, z_t_150m: 15, z_w: 62, z_w_bot: 62, z_w_top: 62) Coordinates: * z_w_top (z_w_top) float32 0.0 1000.0 2000.0 3000.0 4000.0 5000.0 ... * z_t (z_t) float32 500.0 1500.0 2500.0 3500.0 4500.0 5500.0 ... * z_w (z_w) float32 0.0 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 ... * z_t_150m (z_t_150m) float32 500.0 1500.0 2500.0 3500.0 4500.0 5500.0 ... * z_w_bot (z_w_bot) float32 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0 ... * time (time) float64 7.322e+05 7.322e+05 7.322e+05 7.322e+05 ... Dimensions without coordinates: d2, nlat, nlon Data variables: time_bound (time, d2) float64 dask.array<shape=(16401, 2), chunksize=(1, 2)> SST (time, nlat, nlon) float32 dask.array<shape=(16401, 2400, 3600), chunksize=(1, 2400, 3600)> Attributes: nsteps_total: 480 tavg_sum: 64800.0 title: BRCP85C5CN_ne120_t12_pop62.c13b17.asdphys.001 start_time: This dataset was created on 2016-03-14 at 05:32:30.3 Conventions: CF-1.0; http://www.cgd.ucar.edu/cms/eaton/netcdf/CF-curren... source: CCSM POP2, the CCSM Ocean Component cell_methods: cell_methods = time: mean ==> the variable values are aver... calendar: All years have exactly 365 days. history: none contents: Diagnostic and Prognostic Variables revision: $Id: tavg.F90 56176 2013-12-20 18:35:46Z mlevy@ucar.edu $ ```

This is roughly 45 years of daily data, one file per year.

Instead, if I just change decode_cf=True (the default), it takes forever. I can monitor what is happening via the distributed dashboard. It looks like this:

There are more of these open_dataset tasks then there are number of files (45), so I can only presume there are 16401 individual tasks (one for each timestep), which each takes about 1 s in serial.

This is a real failure of lazy decoding. Maybe it can be fixed by #1725, possibly related to #1372.

cc Pangeo folks: @jhamman, @mrocklin