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Re: NetCdf access

> Organization: ASTRO SPOCAE CENTER, Moscow
> Keywords: 199403111044.AA00309

Hi Vitaly,

>   We have got copy of netCDF software from  unidata.ucar.edu
>   It seems from netCDF documents that if I need to access
>   netCDF data I must have mounted directory with netCDF file
>   on may local machine or some way load a copy of netCDF file
>   to  local host. Are we correct in this supposition? Is it
>   possible get data on remote host in XDR format transfer
>   it to a local host and decode data here in local host format.

NetCDF files are stored in XDR format, so they are already in a form that is
independent of machine format.  When you read some netCDF data, it is
converted by the netCDF library to the local (also called "native")
representation at the time the data are read.  The conversion between XDR
form and the native form all happens below the level of the netCDF library
interface, so users need not be aware of the conversion.  To an application
or user of the netCDF library, it appears as if the data is in the native
form of the machine on which the application is running, since the
application only deals with values in the native representation for that
machine.

It is certainly possible to read netCDF data on a remote host.  If a netCDF
file is in a directory mounted from a remote system (via NFS for example),
you can open it and read it exactly as you would read a netCDF file on a
local disk.  The format of the data is independent of what machine it was
written on.  An advantage of this is that the data is portable.  A
disadvantage is that conversion to and from XDR takes place on every data
access, even if you only want to write and read the data on a single kind of
machine.  As explained in the NetCDF USer's Guide introduction, we think the
benefits are worth the cost:

    Translating data into and out of XDR form adds overhead to data
    transfers, but for many applications the extra CPU cycles used to
    convert data to and from a machine-independent representation are not
    significant. The amount of XDR overhead depends on many factors,
    including the data type, the type of computer, the granularity of data
    access, and how well the implementation has been tuned to the computer
    on which it is run. For a large set of applications, the overhead of the
    XDR layer is a reasonable price to pay for portable, network-transparent
    data access.

I hope this answers your question.

You may send specific questions about netCDF to address@hidden if
the questions are not of general interest to the whole netcdfgroup mailing
list.

__________________________________________________________________________
                      
Russ Rew                                              UCAR Unidata Program
address@hidden                                        P.O. Box 3000
(303)497-8645                                 Boulder, Colorado 80307-3000