Creating PAR2 files for UseNet

QuickPar allows you a great deal of flexibility when it comes to creating PAR2 files, but allowances must be made for the capabilities of the newsreaders and the quality of the news-service used by those who download files you may post.

As you are no doubt aware, PAR2 works best if incomplete files are downloaded and used as part of the repair process. Downloading incompletes enables repair to take place in circumstances where (with PAR 1.0) repair would not otherwise be possible.

Unfortunately, some newsreaders do not permit incomplete files to be downloaded at all.

To cater for these users it is recommended that you create and post enough PAR2 files to repair the same number of source files that you would have done with PAR 1.0 files. If you are posting 50 RAR files and would have posted 8 PAR 1.0 files, then you should post enough PAR2 files to allow 8 RAR files to be reconstructed. QuickPar tells you exactly how many files you would be able to repair.

Additionally, since large files have a greater chance of missing at least one article: you should limit the size of the largest PAR2 file. The best option to choose in QuickPar is "Variable (limited to size of largest data file)". This will give you a number of small PAR2 files of varying sizes plus a number of other PAR2 files which contain exactly the right amount of data to repair the largest data file.

The need to split or archive large or variable sized files

PAR 2.0 does not require you to either split large files into smaller files, or combine variably sized files into archives which are then split into equally sized files.

For very large files (e.g. 100MB and upwards) it is recommended that you split the files into smaller parts. Commonly used sizes vary from 5MB to 15MB.

If you use RAR to split files, it is recommended that instead of using Recovery Records, you choose a higher redundancy % in QuickPar. i.e. It is better to set 13% redundancy in QuickPar than 3% RR in RAR plus 10% redundancy in QuickPar. RAR Recovery Records can only repair damage to the RAR file that they are in. Unfortunately errors rend to occur randomly and you may find some RAR files having too much damage for the RRs to repair whilst the RRs in other RAR files go unused. If you create a little extra PAR2 data instead, then they can be used to in the repair of any of the RAR files.

With variablly sized files it is recommended that you do not archive them (unless the individual files are excessibly large).

Getting the block size right

PAR2 files should work perfectly well no matter what block size you choose, but some values for the block size will be much more efficient than others.

It is strongly recommended that you choose a block size that is either equal to or an exact multiple of the article posting size which you plan to use with your news-posting software. When you do this, a single missing article will only require the use of one block of recovery data for repair. If the block size is not an exact multiple of the article size, then it is highly likely that downloaders will need to fetch two recovery blocks to repair one missing article. This means that they are forced to download twice as much recovery data.

QuickPar lets you set your preferred article size in the Options dialogue for both UUEncode (based on 45 bytes per line) and yEnc (based on 128 bytes per line). You should also ensure that the "Restrict block size to multiples of UseNet article size" checkbox is ticked.

If you are posting files that are mostly smaller than the standard article sizes used for large files, then you should also choose a smaller block size. It is recommended that you adjust the block count such that the "Efficiency" rating which QuickPar displays is at least 90%. A rating of 90% means that on average each 100KB of PAR2 data downloaded would allow the reconstruction of 90KB of missing files. You should also choose the "Variable (powers of 2 sizing scheme)" for specifying the size of the PAR2 files.

Creating extra PAR2 files

If you need to create additional PAR2 files, you should ensure that all settings are exactly as they were for the first batch of PAR2 files you created.

You should then adjust the "First recovery block number" to a figure greater than the total number of recovery blocks you have already created. Failure to do this will result in a set of PAR2 files that are duplicates of the ones you have already created.

Posting the PAR2 files

As with PAR 1.0, it is recommended that you post the small PAR2 file called "filename.ext.PAR2" file right at the start (along with any .nfo, .sfv etc files you may be posting). The rest of the PAR2 files should be posted after all of the data files have been posted. This will allow downloaders to check their files as they arrive, and only start downloading PAR2 files once they know how many they will need.

 

Copyright © 2003 Peter B. Clements. All Rights Reserved.