Reading FCI L2 data yields warning on chunking performance

[gerritholl]

Describe the bug

When reading FCI L2 data using the fci_l2_nc reader, a warning is issued about performance of chunk size.

To Reproduce

from satpy import Scene
sc = Scene(
        filenames=["W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-2-CT--FD------NC4E_C_EUMT_20260105085507_L2PF_OPE_20260105084000_20260105085000_N__C_0053_0000.nc"],
        reader="fci_l2_nc")

Expected behavior

I expect no warning or error messages.

Actual results

/home/gholl/checkouts/satpy/satpy/readers/fci_l2_nc.py:185: UserWarning: The specified chunks separate the stored chunks along dimension "number_of_rows" starting at index 4096. This could degrade performance. Instead, consider rechunking after loading.
  self.nc = xr.open_dataset(
/home/gholl/checkouts/satpy/satpy/readers/fci_l2_nc.py:185: UserWarning: The specified chunks separate the stored chunks along dimension "number_of_columns" starting at index 4096. This could degrade performance. Instead, consider rechunking after loading.
  self.nc = xr.open_dataset(

Environment Info:

• OS: openSUSE 15.6
• Satpy Version: main (v0.59.0-95-g579e79709)

Additional context

The warnings should either be suppressed if a false alarm, or they should be avoided by changing how data are chunked upon loading.

[pnuu]

What does ncdump -h <file> say about chunking?

[gerritholl]

ncdump -h <file> does not contain information about chunking, as far as I can see (at least grep has no results for chunk):

netcdf W_XX-EUMETSAT-Darmstadt\,IMG+SAT\,MTI1+FCI-2-CT--FD------NC4E_C_EUMT_20260105085507_L2PF_OPE_20260105084000_20260105085000_N__C_0053_0000 {
types:
  byte enum cloud_type_type {Not\ processed\ \(no\ or\ corrupt\ data\) = 0,
      Cloud\ free\ land\ \(no\ cloud\,\ snow\ or\ ice\) = 1,
      Cloud\ free\ sea\ \(no\ cloud\,\ snow\ or\ ice\) = 2,
      Land\ with\ snow\ or\ ice = 3, Sea\ with\ snow\ or\ ice = 4,
      Cloud\ very\ low\ opaque = 5, Cloud\ low\ opaque = 6,
      Cloud\ medium\ opaque = 7, Cloud\ high\ opaque = 8,
      Cloud\ very\ high\ opaque = 9,
      Cloud\ semi-transparent\ high\ thin = 10,
      Cloud\ semi-transparent\ high\ mean\ thick = 11,
      Cloud\ semi-transparent\ high\ thick = 12,
      Cloud\ semi-transparent\ high\ above\ low\ medium\ cloud = 13,
      Cloud\ fractional = 14, Dust\ contaminated = 15, Dust\ opaque = 16,
      Ash\ contaminated = 17, Ash\ opaque = 18,
      Undefined\ by\ Cloud\ Mask = 19} ;
  byte enum cloud_phase_type {Not\ processed\ \(no\ or\ corrupt\ data\) = 0,
      Water\ cloud = 1, Ice\ cloud = 2, Dust = 3, Ash = 4, Undefined = 5} ;
  byte enum quality_illumination_type {Undefined\ \(space\) = 0, Night = 1,
      Twilight = 2, Day = 3, Sunglint = 4} ;
  byte enum quality_nwp_parameters_type {Undefined = 0,
      All\ available\ \(no\ inversion\) = 1,
      All\ available\ \(inversion\) = 2,
      One\ or\ more\ missing\ \(no\ inversion\) = 3,
      One\ or\ more\ missing\ \(inversion\) = 4} ;
  byte enum quality_MTG_parameters_type {Undefined = 0,
      All\ non-nwp\ available = 1, A\ required\ input\ missing = 2} ;
  byte enum quality_overall_processing_type {
      Not\ processed\ \(no\ or\ corrupt\ data\) = 0,
      Good\ quality\ \(high\ confidence\) = 1,
      Poor\ quality\ \(low\ confidence\ -\ reclassified\ after\ restoral\ tests\) = 2,
      Poor\ quality\ \(low\ confidence\ -\ sunglint\) = 3,
      Poor\ quality\ \(low\ confidence\) = 4} ;
dimensions:
        number_of_rows = 5568 ;
        number_of_columns = 5568 ;
variables:
        int mtg_geos_projection ;
                mtg_geos_projection:long_name = "MTG geostationary projection" ;
                mtg_geos_projection:grid_mapping_name = "geostationary" ;
                mtg_geos_projection:perspective_point_height = 35786400. ;
                mtg_geos_projection:semi_major_axis = 6378137. ;
                mtg_geos_projection:semi_minor_axis = 6356752. ;
                mtg_geos_projection:inverse_flattening = 298.257223563 ;
                mtg_geos_projection:latitude_of_projection_origin = 0. ;
                mtg_geos_projection:longitude_of_projection_origin = 0. ;
                mtg_geos_projection:sweep_angle_axis = "y" ;
        short x(number_of_columns) ;
                x:long_name = "2km x coordinate in mtg_geos_projection for each data point" ;
                x:standard_name = "projection_x_coordinate" ;
                x:unit = "radian" ;
                x:axis = "X" ;
                x:valid_range = 1s, 5568s ;
                x:scale_factor = -5.588778e-05f ;
                x:add_offset = 0.1556195f ;
        short y(number_of_rows) ;
                y:long_name = "2km y coordinate in mtg_geos_projection for each data point" ;
                y:standard_name = "projection_y_coordinate" ;
                y:unit = "radian" ;
                y:axis = "Y" ;
                y:valid_range = 1s, 5568s ;
                y:scale_factor = 5.588778e-05f ;
                y:add_offset = -0.1556195f ;
        cloud_type_type cloud_type(number_of_rows, number_of_columns) ;
                cloud_type:long_name = "Cloud Type" ;
                cloud_type:grid_mapping = "mtg_geos_projection" ;
                cloud_type:coordinates = "x y" ;
        cloud_phase_type cloud_phase(number_of_rows, number_of_columns) ;
                cloud_phase:long_name = "Cloud Phase" ;
                cloud_phase:grid_mapping = "mtg_geos_projection" ;
                cloud_phase:coordinates = "x y" ;
        quality_illumination_type quality_illumination(number_of_rows, number_of_columns) ;
                quality_illumination:long_name = "Illumination condition" ;
                quality_illumination:grid_mapping = "mtg_geos_projection" ;
                quality_illumination:coordinates = "x y" ;
        quality_nwp_parameters_type quality_nwp_parameters(number_of_rows, number_of_columns) ;
                quality_nwp_parameters:long_name = "Quality of NWP parameters" ;
                quality_nwp_parameters:grid_mapping = "mtg_geos_projection" ;
                quality_nwp_parameters:coordinates = "x y" ;
        quality_MTG_parameters_type quality_MTG_parameters(number_of_rows, number_of_columns) ;
                quality_MTG_parameters:long_name = "Quality of MTG parameters" ;
                quality_MTG_parameters:grid_mapping = "mtg_geos_projection" ;
                quality_MTG_parameters:coordinates = "x y" ;
        quality_overall_processing_type quality_overall_processing(number_of_rows, number_of_columns) ;
                quality_overall_processing:long_name = "Overall processing quality" ;
                quality_overall_processing:grid_mapping = "mtg_geos_projection" ;
                quality_overall_processing:coordinates = "x y" ;
        byte product_quality ;
                product_quality:long_name = "Overall Quality of product" ;
                product_quality:unit = "none" ;
                product_quality:_FillValue = -127b ;
                product_quality:missing_value = -127b ;
                product_quality:valid_range = 0b, 100b ;
                product_quality:scale_factor = 0.04f ;
        byte product_completeness ;
                product_completeness:long_name = "Completeness of product" ;
                product_completeness:unit = "none" ;
                product_completeness:_FillValue = -127b ;
                product_completeness:missing_value = -127b ;
                product_completeness:valid_range = 0b, 100b ;
                product_completeness:scale_factor = 1.f ;
        byte product_timeliness ;
                product_timeliness:long_name = "Timeliness of product" ;
                product_timeliness:unit = "none" ;
                product_timeliness:_FillValue = -127b ;
                product_timeliness:missing_value = -127b ;
                product_timeliness:valid_range = 0b, 100b ;
                product_timeliness:scale_factor = 1.f ;

// global attributes:
                :Conventions = "CF-1.7" ;
                :title = "FCI Level 2 Cloud Type Product" ;
                :summary = "The cloud type (CT) product provides a detailed cloud analysis. The CT product contains information on major cloud classes: fractional clouds, semi-transparent clouds, separation of high/medium/low clouds and clouse phase." ;
                :keywords = "" ;
                :keywords_vocabulary = "" ;
                :history = "Original generated version" ;
                :institution = "EUMETSAT" ;
                :location_indicator = "XX-EUMETSAT-DARMSTADT" ;
                :data_designator = "IMG+SAT" ;
                :platform = "MTI1" ;
                :data_source = "FCI" ;
                :processing_level = "2" ;
                :coverage = "FD" ;
                :type = "CT" ;
                :subtype = "" ;
                :component1 = "" ;
                :component2 = "" ;
                :component3 = "" ;
                :product_id = "FCI2CT" ;
                :baseline_version = "" ;
                :release_version = "Original" ;
                :processor_version = "" ;
                :algorithm_version = "" ;
                :format_version = "" ;
                :time_coverage_start = "20260105084000" ;
                :time_coverage_end = "20260105085151" ;
                :processing_mode = "N" ;
                :special_compression = "" ;
                :subsetting = "" ;
                :disposition_mode = "C" ;
                :source = "IDPFI" ;
                :runtime_data = "IDAC RTM TSIT" ;
                :parent_data = "" ;
                :linked_data = "" ;
                :facility_or_tool = "L2PF" ;
                :environment = "ope" ;
                :references = "www.eumetsat.int" ;
                :comment = "" ;
                :date_created = "20260105085507" ;
                :group_tag = "" ;
                :repeat_cycle_in_day = "0053" ;
                :processed_count_in_repeat_cycle = "0" ;
                :count_in_repeat_cycle = "0000" ;
                :instrument_configuration_id = "16384" ;
                :instrument_configuration_id_version = "16384" ;
                :subsettable_groups = "" ;
                :subsettable_groups_present = "" ;
                :mtg_name = "" ;
                :alternative_name = "" ;
                :purpose = "" ;
                :format = "NC4E" ;
                :id = "" ;
                :naming_authority = "" ;
                :creator_type = "institution" ;
                :creator_institution = "EUMETSAT" ;
                :creator_name = "EUMETSAT" ;
                :creator_email = "ops@eumetsat.int" ;
                :creator_url = "www.eumetsat.int" ;
                :license = "https://www.eumetsat.int/legal-framework/data-policy" ;
                :standard_name_vocabulary = "CF Standard Name Table v27" ;
                :project = "MTG" ;
                :time_coverage_duration = "" ;
                :time_coverage_resolution = "" ;
                :cdm_datatype = "" ;
                :date_time_position = "20260105084000" ;
                :time_position = "084000" ;
                :geospatial_lat_min = 0.f ;
                :geospatial_lat_max = 0.f ;
                :geospatial_lon_min = 0.f ;
                :geospatial_lon_max = 0.f ;
}