# Forthcoming update to the format of netCDF files produced by the conversion of GRIB data on the CDS

**URL:** <https://forum.ecmwf.int/t/forthcoming-update-to-the-format-of-netcdf-files-produced-by-the-conversion-of-grib-data-on-the-cds/7772>\
**Category:** C3S - Announcements\
**Tags:** netcdf, cds, grib, era5\
**Created:** [19 November 2024 19:43 UTC](https://forum.ecmwf.int/t/forthcoming-update-to-the-format-of-netcdf-files-produced-by-the-conversion-of-grib-data-on-the-cds/7772 "2024-11-19T19:43:20Z")\
**Posts on this page:** 1\
**Showing post:** 5

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**Author:** ![Kenneth\_Bowman](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/k/ecc23a/32.png) [@Kenneth\_Bowman](https://forum.ecmwf.int/u/Kenneth_Bowman)\
**Post date:** [5 December 2024 21:41 UTC](https://forum.ecmwf.int/t/forthcoming-update-to-the-format-of-netcdf-files-produced-by-the-conversion-of-grib-data-on-the-cds/7772/5 "2024-12-05T21:41:45Z")

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This is my first post to the forum, so my apologies if I am asking in the wrong place.

I have many years of old data in the previous netCDF file format (netCDF 3, 64-bit offset). I have downloaded some data in the new netCDF format (netCDF-4), but reading from the new files is approximately 3 orders of magnitude slower than the old files (5.099 s vs. 0.0084 s).

They both appear to be valid netCDF files and the data looks normal. My only guess is that perhaps the chunk size for the netCDF-4 files is particularly inefficient? I can provide more details if it would be of any help.

Thanks, Ken Bowman

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