Hi,
newer versions of netcdf (tested with 4.2.1 and 4.1.1) allow the
generation of netcdf4 files with shuffling, chunking and compression by
using the attributes:
`_Format', `_Fletcher32, `_ChunkSizes', `_Endianness',_DeflateLevel',
`_Shuffle', and `_Storage'.
Unfortunately, these attributes are not only used for creating the file,
but are also written as normal attributes to the file.
A simple example:
netcdf test {
dimensions:
time = UNLIMITED ; // (2 currently)
variables:
short time(time) ;
time:_Storage = "chunked" ;
time:_ChunkSizes = 1 ;
time:_Shuffle = "true" ;
time:_DeflateLevel = 3 ;
// global attributes:
:_Format = "netCDF-4 classic model" ;
data:
time = 12, 24 ;
}
looks after a 'ncgen -b', and 'ncdump -hs test.nc' like:
netcdf test {
dimensions:
time = UNLIMITED ; // (2 currently)
variables:
short time(time) ;
time:_ChunkSizes = 1 ;
time:_DeflateLevel = 3 ;
time:_Shuffle = "true" ;
time:_Storage = "chunked" ;
time:_ChunkSizes = 1 ;
time:_DeflateLevel = 3 ;
time:_Shuffle = "true" ;
time:_Endianness = "little" ;
// global attributes:
:_Format = "netCDF-4 classic model" ;
}
When using some further processing, e.g. 'nccopy -k 3 -d 9 test.nc
test9.nc', test9.nc will have ambiguos _DeflateLevel settings:
$ ncdump -hs test9.nc
netcdf test9 {
dimensions:
time = UNLIMITED ; // (2 currently)
variables:
short time(time) ;
time:_ChunkSizes = 1 ;
time:_DeflateLevel = 3 ;
time:_Shuffle = "true" ;
time:_Storage = "chunked" ;
time:_ChunkSizes = 1 ;
time:_DeflateLevel = 9 ;
time:_Endianness = "little" ;
// global attributes:
:_Format = "netCDF-4" ;
}
I guess, all those _Attributes should never be written to the file by
ncgen. (Tested with netcdf-4.2.1.1 on linux (ubuntu 12.04 64bit)).
Regards,
Heiko
