NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
Current content can be found at unidata.ucar.edu.
To learn about what's going on, see About the Archive Site.
NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
Current content can be found at unidata.ucar.edu.
To learn about what's going on, see About the Archive Site.
On Apr 25, 2011, at 3:51 PM, John Caron wrote: > On 4/25/2011 1:46 PM, Peter Cornillon wrote: >> >> On Apr 25, 2011, at 3:42 PM, John Caron wrote: >> >>> On 4/25/2011 1:37 PM, Roy Mendelssohn wrote: >>>> yes, internal compression. All the files were made from netcdf3 files >>>> using NCO with the options: >>>> >>>> ncks -4 -L 1 >>>> >>>> The results so far show a decrease in file size from 40% of original to >>>> 1/100 th of the original file size. If the internally compressed data >>>> requests are cached differently than request to netcdf3 files, we want to >>>> take that into account when we do the tests, so that we do not just see >>>> the affect of differential cacheing. >>>> >>>> When we have done tests on just local files, the reads where about 8 >>>> times slower from a compressed file. But Rich Signell has found that the >>>> combination of serialization/bandwidth is the bottleneck, and you hardly >>>> notice the difference in a remote access situation. That is what we want >>>> to find out, because we run on very little money and with compression as >>>> mentioned above our RAIDS would go a lot farther, as long the hit to the >>>> access time is not too great. >>>> >>>> Thanks, >>>> >>>> -Roy >>> >>> in netcdf4/hdf5, compression is tied to the chunking. Each chunk is >>> individually compressed, and must be completely decompressed to retrieve >>> even one value from that chunk. So the trick is to make your chunks >>> correspond to your "common cases" of data access. If thats possible, you >>> should find that compressed access is faster than non-compressed access, >>> because IO is smaller. but it will be highly dependent on that. >> >> John, is there a loss of efficiency when compressing chunks compared to >> compressing the entire file? I vaguely recall that for some compression >> algorithms, compression efficiency is a function of the volume of data >> compressed. >> >> Peter >> > > Hi Peter: > > I think dictionary methods such as deflate get better as the file size goes > up, but the tradeoff here is to try to decompress only the data you actually > want. Decompressing very large files can be very costly. Yes, this is why I chunk. The reason that I asked the question is that this might influence the chunk size that one chooses. Peter > > John > > _______________________________________________ > thredds mailing list > thredds@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: > http://www.unidata.ucar.edu/mailing_lists/ -- Peter Cornillon 215 South Ferry Road Telephone: (401) 874-6283 Graduate School of Oceanography Fax: (401) 874-6283 University of Rhode Island Internet: pcornillon@xxxxxxxxxxx Narragansett, RI 02882 USA
thredds archives: