CODEWITHSUNDEEP
compression7zip
alper
alper

Reputation: 3410

Can `7z` compress only the updated section of the file instead of compressing complete updated file?

First, I created a 1GB file and transfer into /target folder, then I compressed the file using 7z a targer.7z target.

Later I append hello string to the tail of the 1GB file. When I re-compressed the /target folder using update option 7z u target.7z target; I observe that updated file is compressed all over again instead of compressing only its updated section.

[Q] How could I force 7z to compress only the updated section of the file instead of compressing complete updated file? is there any alternative compression methods to achieve this goal?

Example:

$ mkdir target
$ fallocate -l 1G target/temp_1GB_file
$ time 7z a target.7z target
7-Zip [64] 9.20  Copyright (c) 1999-2010 Igor Pavlov  2010-11-18
p7zip Version 9.20 (locale=en_US.UTF-8,Utf16=on,HugeFiles=on,8 CPUs)    
Scanning    
Updating archive target.7z    
Compressing  target/temp_1GB_file
Compressing  target/target.7z    
Everything is Ok

real    0m23.054s
user    0m30.316s
sys 0m1.047s

$ echo 'hello' >> target/temp_1GB_file
$ time 7z u target.7z target # Here complete file has been compressed all over again.
7-Zip [64] 9.20  Copyright (c) 1999-2010 Igor Pavlov  2010-11-18
p7zip Version 9.20 (locale=en_US.UTF-8,Utf16=on,HugeFiles=on,8 CPUs)    
Scanning   
Updating archive target.7z    
Compressing  target/temp_1GB_file    
Everything is Ok    

real    0m23.861s
user    0m30.781s
sys 0m1.192s

Here, as you can see, I appended file with 'hello' string, and instead of compressing 'hello's located file-block and merge with the already compressed 1GB file, complete file has be re-compressed again.

Upvotes: 0

Views: 98

Answers (1)

Mark Adler
Mark Adler

Reputation: 112502

7z is not designed for that.

You can look at the gzlog.h and gzlog.c code for an example of how to append short messages efficiently to a compressed file.

Upvotes: 1

Related Questions