Author Topic: compressing already compressed file  (Read 4636 times)

peter

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compressing already compressed file
« on: March 04, 2015, 05:06:44 AM »
I just read a article about " compressing already compressed file could create a slightly bigger file".  Then, I am just wondering  that how  wanos deals with zipped file ( or already compressed file )  because sending zipped file is not uncommon in this days.

Thanks

ahenning

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Re: compressing already compressed file
« Reply #1 on: March 04, 2015, 06:19:53 AM »
Yes, generally compressing compressed data will introduce overhead. Same for compressing encrypted data. It is the same for all optimization vendors (They may add features to decompress or decrypt certain protocols first).

The larger file is not necessarily always true, since its possible that the file was compressed with a weak algorithm and then compressing the file with a strong algorithm might reduce it further.

What is interesting though is when adding new data to a compressed file that is already transferred, the previous seen data can match on dedup and only the new compressed data is transferred over the WAN.

This can be tested by creating a e.g. 20 MB zip file. Send it across the WAN. Then add a 1 MB file to the zip archive and send it across the WAN again. Depending on the zip program used, the first 20 MB could match on dedup and only the 1 MB new data is transferred.
« Last Edit: March 04, 2015, 06:25:53 AM by ahenning »
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peter

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Re: compressing already compressed file
« Reply #2 on: March 05, 2015, 05:06:00 AM »
Thanks very much. 
I understand network traffic will go through compression and deduplication for data minimization.  I am just wondering which one comes first ( compression or dedup ) for final better reduction rate.

Thanks again

ahenning

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Re: compressing already compressed file
« Reply #3 on: March 05, 2015, 10:19:45 AM »
First dedup, then compression for best reduction rate.
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