Hi Charlie, > A user wants to uncompress their files with NCO and setting the > deflation level to zero works yet the variables are still listed > as having the deflation filter set. Now I make NCO manually turn-off > the deflate flag when the deflation level is zero yet I'm wondering > why the netCDF library does not do this automatically when I call > var_deflate() with level 0. This makes me think I don't fully > understand what deflation level equals zero means. And I'm wary > of introducing bugs through naÃvete. I'm CC:ing hdf5 support, in case they have a correction for my understanding of why deflation level 0 is distinguished from uncompressed. I think netCDF-4 is following HDF5's meaning for deflate level 0. The HDF5 reference documentation for H5P_set_deflate() says: Note that setting the compression level to 0 (zero) does not turn off use of the gzip filter; it simply sets the filter to perform no compression as it processes the data. > Is there any distinction between uncompressed variables and variables > with compression set to level zero? I think this means that the variable must still use chunked rather than contiguous layout, whereas an uncompressed variable could use contiguous layout. > Are such variables fully uncompressed? Yes. > Perhaps this could be indicated in the manual. Right, if hdf5 support confirms my understanding, I'll make this difference explicit in the documentation. > Is there a reason not to unset the deflation flag when deflation > level is zero? I'll pass the buck to HDF5 support, in case they know a good reason why the deflation property is set for data "compressed" with deflation level 0. --Russ Russ Rew UCAR Unidata Program address@hidden http://www.unidata.ucar.edu Ticket Details =================== Ticket ID: WNT-807985 Department: Support netCDF Priority: Normal Status: Closed
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