In order to take full advantage of PAR2 files posted on UseNet, it is important to configure your newsreader to permit you to download incomplete files. PAR2 is still useable if your newsreader does not permit this, but you will need to download a greater quantity of PAR2 files than you would otherwise and repair will take considerably longer as a result. Reconstructing a whole file with PAR2 will always take longer than PAR1 would. Repairing a damaged file using PAR2 will very probably take less time than PAR1 would take to reconstruct it.
Should PAR2 files be downloaded before, after or at the same time as data files?
It is normally best to download files in the same order that they are uploaded. This will reduce the chance of individual files expiring before your newsreader makes the attempt to fetch them.
As with PAR 1.0, posters should normally post the very small PAR2 file before the data files, and the other PAR2 files (which actually contain recovery data) after the data files. If the poster has posted all of the PAR2 files before the data files instead of after them and your newsservice has a very low retention time, then make sure that you download ALL of the PAR2 files first to avoid any possibility that they might expire before you discover than you need them.
You should always download the smallest PAR2 file first (the one called "filename.ext.par2") since you can use this to verify the other files that you download. This works in exactly the same was as the small .PAR file for PAR 1.0. If you want to, whilst the other files are being downloaded you can use the Monitor function to scan them as soon as they finish downloading.
What should you download when you can see that there are incomplete files before you start?
As always with PAR2, it is important to queue ALL incomplete files to be downloaded in addition to the complete ones. This will maximise the amount of good data available to QuickPar which will in turn reduce the amount of time that it takes to repair.
You may then wish to estimate how much recovery data you need to download and queue enough PAR2 files for download at the same time.
Providing that the poster has set the PAR2 block size equal to the article posting size, then you should be able to get an accurate estimate of what you need simply by adding up the total number of missing articles. NB however that if the uploader has not matched the article size to the block size they used to create the PAR2 files, you might require up to two times as many recovery blocks as there are lost articles.
Once you know how many recovery blocks you need you can work out which PAR2 files to queue for download. See the answer to the next question for details of how to work out which PAR2 files to download to obtain a specific amount of recovery data.
If the posted PAR2 files have enough recovery blocks, then repair should be possible.
QuickPar says I need NNN recovery blocks, what should I download?
NB A "Recovery Block" and a "Recovery File" are NOT the same thing. If you are told you need 20 recovery blocks, then that does not mean you need to download 20 PAR2 files.
PAR2 files normally contain more than 1 recovery block and the sizes of the PAR2 files vary in a way that makes it possible to download a combination that exactly matches the number of recovery blocks that you require.
PAR2 files have names that take the following form:
The number NNN tells you exactly how many recovery blocks are contained within the PAR2 file. An example of a set of PAR2 files would be:
If you were told that you needed 60 recovery blocks, you could either download two of the "+40" files (giving you 20 more blocks than you need), or one "+40" file and the "+28" file (giving you 8 more blocks than you need), or even one "+40" the "+15", "+04", and "+01" files (giving you exactly 60 recovery blocks).
What to do if you have downloaded all incomplete files and all available PAR2 files but you are told you still need more recovery blocks
As with PAR 1.0, when this happens you can either ask for individual article fills, whole file reposts, or extra recovery blocks. It is recommended that you ask for more PAR2 files to be posted.
It is extremely important when asking for additional PAR2 files that you specify exactly how many Recovery Blocks you need. If all you say is something like "please post 2 more PAR2 files" then the uploader won't know how many Recovery Blocks to post.
It is recommended that when making your request that (in addition to specifying how many more recovery blocks you need), you should include a comment confirming that you have downloaded all available incomplete files and PAR2 files. You may wish to state how many blocks were missing from the incomplete files. Posters are more likely to go to the trouble of posting extra PAR2 files if they are sure that you have already made every effort to make full use of what has already been posted.
When does using PAR 2.0 beat PAR 1.0?
Using PAR2 files beats PAR1 files under any of the following circumstances:
- When an apparently complete file has been downloaded that turns out to be damaged. With PAR 1.0 you must download an extra PAR file, but with PAR 2.0 you need only download a small PAR2 file.
- When a large number of files are incomplete. With PAR 1.0 there are unlikely to be enough PAR files available to enable the incomplete files to be fully reconstructed. With PAR 2.0 you would download all of the incomplete files plus a small quantity of PAR2 files with which the incompletes would be repaired instead.
- When the posted files are of a varied sizes and one of the smaller files is missing or incomplete. With PAR 1.0 you must download a PAR file that is as large as the largest of the files posted. With PAR 2.0 you only need to download PAR2 files equivalent to the size of the small file (for a complete reconstruction) or less for a repair