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Creating PAR2 files for UseNet newsgroups Top

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.

To get the most benefit from the use of PAR2, it is important that everyone ensure that they download incomplete files. Downloading incomplete files enables a repair to take place in circumstances where (with PAR 1.0) repair would not otherwise be possible. When all available incomplete files have been obtained, QuickPar will have the maximum possible amount of good data available and this will minimize the amount of repair required (and hence the time that QuickPar takes to repair the damage).

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.

It is however recommended that you do split large files (e.g. 50MB and upwards).

The reason that splitting large files is recommended is mainly because of the fact some newsreaders and newservices, do not permit users to download files with missing articles as incomplete files. The larger the file, the greater the effective loss this causes and the greater the amount of PAR2 files that would be required for a repair. Additionally, some newsreader programs operate better when downloading multiple small files than they do with single large files (because of the way they allocate file and article downloads when using multiple connections to newsservers).

Commonly used sizes vary from 5MB to 15MB when posting files up to 800MB, and a split size of 50MB is frequently used when posting DVD content.

You can have QuickPar split the files for you at the same time that it creates the PAR2 files. Just click the "Split Files" checkbox and enter a figure for the size you want to limit the splits to. The size you enter will be used as a guide for the actual size used. The actual split size will be an exact multiple of the block size. This ensures that all articles posted to UseNet contain a whole block.

If you are posting files that can be compressed, you may wish you use a program such as WinRAR to do this. If you use WinRAR to compress the files before creating PAR2 files, you should ensure that you do NOT use WinRAR's file splitting function, and also do NOT create Recovery Records. You should create a single large RAR file, use that RAR file with QuickPar, and have QuickPar handle the splitting.

If you use a basic splitter program (such as HJSplit, MasterSplitter etc), then you should create the PAR2 files from the original unsplit file rather than from the split files. This has the advantage that downloaders do not need an unsplitter program because QuickPar will join the splits.

If you don't use QuickPar's built in file splitting function, then it is recommended that you set the split size to be an exact multiple of the PAR2 block size (which should of course be equal to the UseNet article size you are using). This has the advantage that every article posted contains a whole block of data, reducing the total number of articles that have to be posted (and therefore also reducing the chances of lost articles causing too much damage).

With multiple files (and especially with variably sized files) it is recommended that you do not archive them (unless the individual files are excessively large). Posting the original files permits the downloaders to choose not to download all of the files if they wish, and makes the files immediatly useable after download without the need to unarchive them first.

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.

NB Many news services do not propogate very large article sizes. Virtually all services handle article sizes up to 500KB in size and most handle articles as large as 1MB. You should set your news posting software and the block size in QuickPar appropriately.

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). Commonly used values include 3000 lines for yEnc (giving a block size of 375KB) and 8000 lines for UUEncode (giving a block size of just over 351KB).

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.

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.

Creating extra PAR2 files

If you need to create additional PAR2 files, you should start by having QuickPar open your existing PAR2 files. Once QuickPar has verified your files, click the "Extra" button to go to the PAR2 creation window. You will find that all of the details for creating new PAR2 files are already filled in and you cannot change them. The only setting you will be able to change is the number of new recovery blocks you want to create and how many PAR2 files you want them split into.

Some newsreaders do not permit incomplete files to be downloaded.

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.




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