# Virtual Data Set

**URL:** <https://forum.hdfgroup.org/t/virtual-data-set/9206>\
**Category:** HDF5 Library\
**Created:** [November 27, 2021, 3:53pm UTC](https://forum.hdfgroup.org/t/virtual-data-set/9206 "2021-11-27T15:53:04Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![bogdan.oancea.ext](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/b/a5b964/32.png) [@bogdan.oancea.ext](https://forum.hdfgroup.org/u/bogdan.oancea.ext)\
**Post date:** [November 27, 2021, 3:53pm UTC](https://forum.hdfgroup.org/t/virtual-data-set/9206/1 "2021-11-27T15:53:04Z")

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Hi,

We are prototyping spitting a large hdf5 file into multiple h5 files by using Virtual Data Sets. For our application, we need to return an error in case the caller tries to read data from a VDS and some of the referenced files that store the requested data are not available.

For example, consider a VDS contains a 3 rows and the VDS splits data by rows, Assume the refenced files for row 1 and 2 are available but the file containing rows 3 is not available.  
We would like to read data with any chunk size from chunks containing data from rows 1 and 2 and get an error when trying to read data from chunks containing data from row 3 (for example, if reading data by columns with chunks 1x3).

Is it possible to specify either when creating the VDS or when opening a hdf5 for reading that HDF5 library should return an error for missing data from VDS files rather than using the VDS fill value?

Thanks,  
Bogdan

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**Author:** ![gheber](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/gheber/32/1919_2.png) [@gheber](https://forum.hdfgroup.org/u/gheber)\
**Post date:** [November 29, 2021, 10:25pm UTC](https://forum.hdfgroup.org/t/virtual-data-set/9206/2 "2021-11-29T22:25:08Z")

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Bogdan, I think VDS was designed with a “forgiving” mindset w.r.t. missing elements, and I don’t see a way to change the error behavior. `H5Pset_virtual_view()` lets you control how aggressive you want to be in the substitution of missing values for missing mapped elements, but this doesn’t affect the error behavior.

Since a VDS is essentially metadata, you can do manually what you set out, i.e., through a combination of `H5Pget_virtual_count()`, `H5Pget_virtual_filename()`, etc., you can discover what is or isn’t available. This will be particularly tedious if some of your VDS contain a mixture of non-virtual and virtual datasets. Otherwise, this shouldn’t be too bad. A dimension with an indefinite extent combined with `printf` style file definitions might give you another headache, but in the end, only you can know what’s really there.

Best, G.