Re: [thredds] How are compressed netcdf4 files handled in TDS

Hi John:

On Apr 25, 2011, at 12:42 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.

Hi John:

But do you cache the entire chunk that you decompress, or do you toss it?  So 
if I make a second request that has data in that chunk i it saved or is it 
reread from the file.



"The contents of this message do not reflect any position of the U.S. 
Government or NOAA."
Roy Mendelssohn
Supervisory Operations Research Analyst
Environmental Research Division
Southwest Fisheries Science Center
1352 Lighthouse Avenue
Pacific Grove, CA 93950-2097

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