Re: [netcdfgroup] NetCDF external links

If I understand it correctly, you are correct: from the API
point of view, it is invisible. But that is not the issue for me.
The issue is that now one needs multiple files in order to acccess
data rather than having everything kept in a single file. That means,
for example, that if one of those underlying files is missing,
then I assume the whole dataset is unreadable.
=Dennis Heimbigner
 Unidata


On 7/13/2016 3:25 PM, Roy Mendelssohn - NOAA Federal wrote:
Hi Dennis:

I asked about this also awhile back.  The key question to my mind is how the 
virtual dataset works in HDF5.  Perhaps the interface is such is that it is 
transparent to higher level calls, much like when you create virtual datasets 
in TDS.  In that case, the calls would be to as if there was one file.

-Roy
On Jul 13, 2016, at 2:09 PM, dmh@xxxxxxxx wrote:

To this point in time, it has been an implicit assumption
that a netcdf dataset encompasses a single file. This functionality
effectively challenges that assumption. So, I would solicit
community input on this. What problems are created if a netcdf
dataset now requires multiple files?

=Dennis Heimmbigner
Unidata


On 7/13/2016 5:09 AM, Eugen Betke wrote:
Dear NetCDF-Group,

we have been working on NetCDF external link functionality. This allows
NetCDF applications to create dimension variables which values are
stored in an external file. Therefore, it uses the HDF5 virtual dataset
(VDS) functionality. This is useful for, e.g., climate applications that
rely on a variable per file and timestep configuration. The idea is to
store the grid in a separate file and link our data to this grid. We
already have our first working version. You find the patch and the
examples on our page:

http://wr.informatik.uni-hamburg.de/research/projects/bullio/netcdf_external_links/start


Under the hood it uses HDF5 virtual datasets. VDS has the advantage of
being compatible to the functions that are supported by oridinary
datasets. Therefore, files containing VDS should be supported by the
most software.

There is a minor issue related to HDF5, the call H5F_try_close function
fails, when ncdump trys to read data from an external dimension. So far
we found a workaround, but we will fix this issue.

It would be great if external link functionality could be supported by
netCDF at some timepoint. We would like to improve our patch and for
that reason we need your feedback. If you have some idea to the issue
above, we would be grateful for each hint.

Regards,
Eugen

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NOAA/NMFS
Environmental Research Division
Southwest Fisheries Science Center
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