PAR2 files contain error-correction data that can be used to fix files that become corrupted during the upload/download process on Usenet where there is no built-in error correction. PAR2 files work on both Windows and Mac, and can be used to protect any kind of file.
PAR2 files don’t have a separate life of their own. You don’t decompress them like you do RAR files. PAR’s only exist to fix other files. Sometimes in a newsgroup you will see a set of PAR’s posted with no other files. The poster did that as a favor to somebody who needed to fix a file that he had downloaded previously. So, you don’t need to download PAR files unless you need them to fix a damaged file in your possession. Of course, it is not always obvious if a file is damaged. For example, there could be small flaws in a video that you can’t find unless you watch the whole thing very closely. It is not a bad idea to download the PAR’s to make sure that your files are OK.
PAR2 files are able to magically fix, and even re-create missing files. Suppose you download 9 of the 10 RAR’s from a binary newsgroup that contain a video file, and you can’t find the 10th. If the poster included some PAR files, you can download them and actually re-create the missing file! If the poster did not include any PARs, as newbie posters often do, you could post a message in the group and ask somebody who was able to get all the files to make, and post some PARs for you. Anybody can make PARs as long as they have a good copy of the files in question.
There are two generations of PAR technology in use: PAR and PAR2. PAR filenames look like this:
somefile.p01
somefile.p02
somefile.p03
PAR2 filenames look like this:
somefile.par2
somefile.vol000+01.PAR2
somefile.vol001+02.PAR2
somefile.vol007+05.PAR2
To use PAR files, open the first PAR or PAR2 with the free PAR decoder in SuperNZB. SuperNZB will use the PARs to examine the target files (RAR files, AVI files, etc.) and fix them if needed.
Both PAR and PAR2 do the same thing, but PAR2 is much better. Look at the PAR2 filenames above. The ‘+01′, ‘+02′, and ‘+05′ indicate how many ‘blocks’ each file has, and that can save you download time. To do that, you would first download the associated files whether they are RARs, AVIs, etc. Then you would download the file that ends with ‘.par2.’ Run that file through your un-PARing program, and it will tell you how many more blocks you need, if any. If you needed seven blocks, then you could download the second and third PAR files. If you only needed two blocks, then you could just download the second file.
PAR files cannot contain viruses, so there is no risk in using them.
I have two problems with SuperNZB one that makes the program almost useless:
At the end of a particular download the DL stops due to a message that informs me the par2 file is not in my downloads folder. Nonsense! Every time I check the file is there.
1. I setup files for overnight downloading, yet I only get one particular set of files because of this error, when I should get ten or twelve.
During the day I have to check every half hour or so to see if my download has stopped.
Is there anyway I can turn the Par Manager off?
2. When I add a new set of files that I want immediately it is impossible without highlighting the files and dragging them to the top of the queue. Depending on how many files are in the queue this is a long, slow process.
Toast Titanium has had the “Send to Top or Bottom” feature for at least a half dozen generations. Do you plan to implement this feature anytime soon?
Thanks for your feedback,
Lonnie
You can stop SuperNZB from starting the PAR process by going onto the “File” menu, then to “Open Par Job Manager”, and leaving that window open. You don’t have to setup anything; just let it sit there. As long as it is open, SuperNZB understands that you don’t want it to do its default PAR processing.
The Par program needs to be in the same folder as the SuperNZB program, not the downloads folder. However, even if it is present, there are other things that could prevent the operating system from launching the app. We can look into that further if you want, though the solution above will prevent that error message.
A “Send to top or Bottom” feature is a good idea. I have added it to the list of possible new features for the next version.
Matt
I use pars and of course I love them. Even broken PAR2 can be still usable! One thing I don’t understand to this day and can’t seem to find any explanation is as to how PAR2 works exactly.
The way I look at PAR2 and RAR is like this:
rar = a wall. Now if that wall has a hole, then
par = a brick that can patch any hole in the wall, but…
How the hell that same brick can fit ANY hole!? That’s the real magic here!
How the hell one block can retain information for the entire set of rars!???
Let’s use the smallest unit.
Let’s say I’ve got a set of archive that is 100 rars long with 10% of pars.
100 users download the same set off the same source, and each of them has an error in a different rar.
Now just ONE rar is broken and requires only ONE block of par in order to fix it.
Here is what baffles me. Each of the 100 users can download just 1 block off the entire par2 set (that just 1% of the whole rar set information) and that 1 magical block will fix any damage in a different spot in each of the 100 pars!!!!! ( as long as it’s just 1 block of course)
they don’t even need to download the whole set, which means par set is not some kind of compressed image of the rar set!
HOW IS THAT POSSIBLE!?… MAGIC!!!?
adb0rg,
It sure does look like magic, however the Reed–Solomon algorithm has been around for a long time. I haven’t studied it myself, but it looks pretty complicated on the Wikipedia page. If you come across a simple explanation please let me know.
Matt