Re: [netcdfgroup] Unexpectedly large netCDF4 files from python

Two things come to mind.
1. I assume you are writing a netcdf-4 (enhanced model) file. There is overhead associated with the file such that even if you wrote no data, it would have a
    noticable size. However, it should not be 73 MB!

2. Do you have fillvalue set and are there other, large variables that you did not write? If so, then they will implicitly be filled with the fillvalue and written out.
=Dennis Heimbigner

On 4/5/2016 12:44 PM, Val Schmidt wrote:
Hello netcdf folks,

I’m testing some python code for writing sets of timestamps and variable length binary blobs to a netcdf file and the resulting file size is perplexing to me.

The following segment of python code creates a file with just two variables, “timestamp” and “data”, populates the first entry of the timestamp variable with a float and the corresponding first entry of the data variable with an array of 100 unsigned 8-bit integers. The total amount of data is 108 bytes.

But the resulting file is over 73 MB in size. Does anyone know why this might be so large and what I might be doing to cause it?



from netCDF4 import Dataset
import numpy

f = Dataset('scratch/','w')

dim = f.createDimension('timestamp_dim',None)
data_dim = f.createDimension('data_dim',None)

data_t = f.createVLType('u1','variable_data_t’)

timestamp = f.createVariable('timestamp','d','timestamp_dim')
data = f.createVariable('data',data_t,'data_dim’)

timestamp[0] = time.time()
data[0] = uint8( numpy.ones(1,100))


Val Schmidt
University of New Hampshire
Chase Ocean Engineering Lab
24 Colovos Road
Durham, NH 03824
e: vschmidt [AT] <>
m: 614.286.3726

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