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.
>>> 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
> 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.
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