[netCDF #PBW-682100]: nccopy deflation vs API deflation

Hi Nick,

> I am running some tests with a code I am converting from using a flat
> file to netcdf/hdf5. I am using the parallel MPIIO access mode so unable
> to use the deflation calls via the API. I thought I would use nccopy
> -d9 as a post process on my files to compress them and therefore get
> some space saving whilst still retaining the ability to do a parallel
> read in other related codes.
> 
> However, I find that I get quite poor compression using nccopy, much
> worse than I get if I use the API call. In some cases, nccopy -d9 gives
> little or no compression whilst using the API gives me 4-5x compression.
> 
> Is this something you would expect or am I missing something critical
> in this case?

No, you should expect exactly the same compression using nccopy as with the API 
calls.
nccopy calls the API for each variable in the file with whatever compression 
level you 
specify.  The API calls are somewhat more flexible, in that you can specify a 
differnt level
of compression (or no compression) for each variable separately, but if you use 
the same
compression for every variable, there should be no differencce.

If you are seeing something different, it sounds like a bug.  Can you provide a 
sample
file that we could use to reproduce the problem and diagnose the cause?

--Russ

Russ Rew                                         UCAR Unidata Program
address@hidden                      http://www.unidata.ucar.edu



Ticket Details
===================
Ticket ID: PBW-682100
Department: Support netCDF
Priority: Normal
Status: Closed