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Author Topic: How to add recovery record to ZIP or 7-Zip archive  (Read 5963 times)
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Yutaka Sawada
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« on: 2012 Feb 16, 06:51:46 pm »

 ZIP archive (extension .zip) is used widely and 7-Zip archive (extension .7z) is useful. Because they don't have a feature of recovery record, someone might feel RAR archive is better. But, by using Parchive, normal ZIP or 7-Zip archive can be protected with recovery record also.

Step 1:
Create a ZIP or 7-Zip archive file.

Step 2:
Create a single PAR2 file for the archive file.

Step 3:
With binary editor or Copy command, append the PAR2 file to the archive file.

 Then, the appended archive file contains recovery record. The construction of the file is like below;
[ Original archive file ][ Appended PAR2 recovery data ]

 Because the file header is still the one of archive, it is treated as archive file, and it can be extract with normal archiver software. Because the file contains PAR2 packets, PAR2 client can treat it as PAR2 file also. As MultiPar can treat a PAR2 file which has different extension from ".par2", Drag&Drop the protected archive onto MultiPar short-cut icon will verify the archive file. On the MultiPar GUI, the archive with recovery record is shown as "Appended" status, when the original part of archive is complete.

 Note, if you repair the archive, recovery record (appended PAR2 recovery data) will be removed. When you want to keep the PAR2 recovery data, you need to copy the file at first, and rename it to "something.par2", then open the "something.par2" by MultiPar (or QuickPar). Though you can split a protected archive into original part and recovery record by a binary editor, it is not easy task for normal users.

 I think this method is useful to protect only one archive file. It is same as having two files (an archive file and its PAR2 file), but is convenient for transport / public / distribution as one ZIP/7-Zip file. Normally freeware / shareware publication site accepts only one archive file. If an archiver software has a built-in feature to append PAR2 recovery data to a created archive, it may be useful. I wrote a sample batch script to do this task, which is included in MultiPar's help.

* batch.htm (15.72 KB - downloaded 868 times.)
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Brat.T
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« Reply #1 on: 2012 Feb 18, 10:52:46 pm »

I was always wondering why the context menu shows verify entry even when the file is not a parchive. So that's the reason.
Really great.
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badon
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« Reply #2 on: 2012 Feb 25, 05:49:13 pm »

Interesting. I wonder if the 7zip author can use MultiPar to add recovery features to 7zip?
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badon
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« Reply #3 on: 2012 Jun 30, 07:18:47 pm »

The FreeArc developer plans to implement a Reed-Solomon recovery record. I made a post that mentions this one:

https://sourceforge.net/apps/phpbb/freearc/viewtopic.php?f=3&t=215

Maybe FreeArc will be the first file archiver to be officially compatible with MultiPar?
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Yutaka Sawada
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« Reply #4 on: 2012 Jul 03, 05:26:31 pm »

 I'm not sure that appending PAR2 recovery data is good for a new archiver format. From a view of compression, PAR2 file is redundant and inefficient. While the redundancy is good for recovery, it is bad for compression rate. Using PAR2 file is an easy solution, but is not the best way in a special file format.

 A good point of PAR2 file is common format. To use recovery record of WinRAR, you need to use WinRAR (non freeware). Though RSC32 or ICE ECC is good for recovery, they are usable on Windows OS only. There are many PAR2 client for many OSes, and all users can append a PAR2 file onto any file freely. And other users can verify/repair with the PAR2 recovery data. Thus, this method will be suitable for widely used major file format.
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badon
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« Reply #5 on: 2012 Jul 03, 07:32:05 pm »

Right, the whole idea behind using PAR2 for recovery data in an archive format is that it is a standard, accessible to anyone, using a wide variety of tools. If archivers like FreeArc become capable of adding PAR2 recovery data to their archives, it will be much easier for people to both compress their data, and protect it from corruption.

The most important reason to do that is for user-friendliness. How many people do you know that are willing and able to append binary file data manually? I don't know anyone, not even at the universities. No matter how technically competent a person is, user-friendliness is always an improvement, and it will ensure that MultiPar and other implementations of Parchive technology get the wide use they deserve.
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Backup is not enough. Protect your data with MultiPar.
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« Reply #6 on: 2013 Sep 16, 12:42:03 am »

Have you ever thought about using tar? I'm not sure how it affect winzip, but I use 7z alot and what I've done as a test was to tar the two files together, then rename the tar file back to a 7z file. There is a slight overhead, about 100k, but not much. When opening as a 7z file, it will show the original pdf file. When opening as a tar file, it will show the 7z + par2 files. And when opening as a par2 file, well... it doesn't say "Appended" yet  Wink but no parts are damaged and repair is possible as well.
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Yutaka Sawada
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« Reply #7 on: 2013 Sep 18, 10:29:22 pm »

 I never use TAR file format. Though it seems to be a non-compressed archive, I don't know the encoding mode. You may use whatever non-encoded archive format to join some files. It is same as putting both source files and PAR2 files in a ISO-image for CD-R, or putting them in a non-compressed ZIP file.

 But, there is a problem of ignoring small source files in the archive. MultiPar (and QuickPar) won't search tiny file (smaller than block size) in an archive file, because it is slow and inefficient in PAR2. (I plan to solve this problem in PAR3.)
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