Phil,
I have been using a kludgy method to manually check the extra padding,
because there is no direct query in the current netcdf library. The
method is to regenerate the whole file with ncdump and ncgen, and then
simply see how much the file size is reduced. If this is of any
interest, here is a copy of my current shell script:
http://www.esrl.noaa.gov/psd/people/dave.allured/data/netcdf/scripts/padsize
--Dave
On Wed, Jul 11, 2012 at 3:45 AM, Bentley, Philip
<philip.bentley@xxxxxxxxxxxxxxxx> wrote:
> Hi folks,
>
> My current understanding is that if one increases the length of a netCDF
> attribute then this will trigger a costly rewrite operation on the whole
> file since the header section is of fixed size. At least for netCDF-3 files,
> I think.
>
> But does this file rewrite penalty apply if the *net* change in header
> length arising from a series of attribute changes is zero or less? Put
> another way, is the basic constraint the overall length of the header
> section, or the length of the individual attribute that one is planning to
> change ?
>
> Some simple tests using ncatted suggest that it's the former - the length of
> the header section - though it would be useful to have this confirmed or
> refuted.
>
> For example, I make changes to a series of text attributes such that they
> are now all shorter in length. This presumably frees up space in the header
> section. I then increase the length of some other attribute (perhaps a
> vector of ints). But that increase in length is accommodated by the space
> freed up earlier. So in this case a file rewrite/copy operation is not
> triggered, right?
>
> Lastly, I notice that it's possible to add some padding to the header
> section using the h_minfree parameter to the nc__enddef() function. Is it
> possible therefore to query the current amount of free space in the header
> section? I couldn't immediately see a function for doing that.
>
> TIA,
>
> Phil
>
>
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