Due to the current gap in continued funding from the U.S. National Science Foundation (NSF), the NSF Unidata Program Center has temporarily paused most operations. See NSF Unidata Pause in Most Operations for details.
Due to the current gap in continued funding from the U.S. National Science Foundation (NSF), the NSF Unidata Program Center has temporarily paused most operations. See NSF Unidata Pause in Most Operations for details.
Hi John, Thanks for this. > netcdf-3 IOSP uses a bufferred RandomAccessFile implementation, default > 8096 byte buffer, which always reads 8096 bytes at a time. the only > useful optimisation is to change the buffer size. Good to know, thanks. I would have thought that this would mean that there's no point reading data of less than 8096 bytes. But in my tests I see that even below this value there's a linear relationship between the size of data being read and the time to read the data (i.e. it's quicker to read 4K than 8K). I don't quite understand this. Are there any specs for the NetCDF-4 format that I could read? I'd like to know more about how the data are compressed, and how much data actually need to be read from disk to get a subset. Cheers, Jon -----Original Message----- From: netcdf-java-bounces@xxxxxxxxxxxxxxxx [mailto:netcdf-java-bounces@xxxxxxxxxxxxxxxx] On Behalf Of John Caron Sent: 15 July 2010 00:26 To: netcdf-java@xxxxxxxxxxxxxxxx Subject: Re: [netcdf-java] Reading contiguous data in NetCDF files Hi Jon: On 7/14/2010 2:51 PM, Jon Blower wrote: > Hi, > > I don't know anything about how data in NetCDF files are organized, but > intuitively, I would think that, for a general 2D array, the data at > points [j,i] and [j,i+1] would be contiguous on disk. Is this right? > (i is the fastest-varying dimension) > yes, for variables in netcdf-3 files > I might also suppose that, for an array of size [nj,ni], that the data > at points [j,ni-1] and [j+1,0] would also be contiguous. Is this true? > yes, for variables in netcdf-3 files that dont use the unlimited dimension > If so, is there a method in Java-NetCDF that would allow me to read > these two points (and only these two points) in a single operation? > netcdf-3 IOSP uses a bufferred RandomAccessFile implementation, default 8096 byte buffer, which always reads 8096 bytes at a time. the only useful optimisation is to change the buffer size. > (Background: I'm trying to improve the performance of ncWMS by > optimising how data is read from disk. This seems to involve striking a > balance between the number of individual read operations and the size of > each read operation.) > > Thanks, > Jon > > -- > Dr Jon Blower > Technical Director, Reading e-Science Centre > Environmental Systems Science Centre > University of Reading > Harry Pitt Building, 3 Earley Gate > Reading RG6 6AL. UK > Tel: +44 (0)118 378 5213 > Fax: +44 (0)118 378 6413 > j.d.blower@xxxxxxxxxxxxx > http://www.nerc-essc.ac.uk/People/Staff/Blower_J.htm > > > _______________________________________________ > netcdf-java mailing list > netcdf-java@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: http://www.unidata.ucar.edu/mailing_lists/ > _______________________________________________ netcdf-java mailing list netcdf-java@xxxxxxxxxxxxxxxx For list information or to unsubscribe, visit: http://www.unidata.ucar.edu/mailing_lists/
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