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

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 



On Apr 25, 2011, at 12:28 PM, John Caron wrote:

> On 4/25/2011 11:30 AM, Roy Mendelssohn wrote:
>> Hi All:
>> We just converted one or our larger datasets  (larger in terms of the number 
>> of files that are aggregated) into compressed netCDF4. There is a 
>> substantial savings in storage, but we wanted to do a series of tests to see 
>> what hit in access time we would take, if any, wsince many of our users will 
>> make requests involving a lot of time periods.
>> In order to design these tests properly, we need to get a better 
>> understanding of how the TDS handles netcdf4 datasets that have compression. 
>>  Are the decompressed data cached, or more accurately cached any differently 
>> from data read from an uncompressed series of netcdf3 files, or since the 
>> decompression is handled automatically on the read, is everything handled 
>> the same after that?
>> We would also be interested other peoples experience with compressed netcdf4 
>> files in TDS, in particular when the extracts are not synoptic, but cover a 
>> lot of time periods in a region, or make a lot of very small calls to a 
>> large number of time periods  - such as we need to do for tagging data.
>> Thanks for any info,
>> -Roy
> Hi Roy:
> I assume you mean internally compressed, not externally (like zipping up a 
> file) ?
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Roy Mendelssohn
Supervisory Operations Research Analyst
Environmental Research Division
Southwest Fisheries Science Center
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