To decrease the amount of fills, I'm using parity volume sets.

What's this?

Let's say, I have 20 rar volumes (foobar.r00 to .r18 + .rar). With the
appropriate program I can create a parity volume set for those files.
With a setting of 10 % redundancy, 3 files will be created:

foobar.P01
foobar.P02
foobar.PAR

This is a parity volume set.
The .Pxx files are called parity volumes. They contain a parity
checksum of the files.
The .PAR file is called index file. It contains information, what
files are stored in the parity volume set and their checksums.


What's it for?

If a file stored in a parity volume is lost, you can restore
it out a parity volume and the remaining files. Case in point,
you are missing .r04. Then you can restore it out of .r00 to .r03,
.r05 to .rar and one Pxx file. It doesn't matter, which Pxx file it is
(p01/p02/p03) - everyone will do. If you miss 2 files, you can restore
them with 2 parity volumes. (Again, it doesn't matter which ones...)
So you can consider the Pxx files as "jokers". Everyone can restore one
missing file.


When does it not work?

If you're missing more files than parity volumes are available, you can't
recover them.
So you have to get more files or more parity volumes.


What do I have to do?

Step 0: get the program

Download at:
www.wehlus.de/mirror
Read the manual and play a bit with it to see how it works...


Step 1: ignore it
Download only the rar files and the sfv (as usual).


Step2: try to get your files 

If a file is incomplete or corrupt, do the usual things.
(waiting a bit, trying other servers, trying the recovery record etc.)


Still missing one? (usually the time for a fill request):
Step 3: determine, how many files you miss 

Do this the usual way or download the index file (PAR).
It's small and works like a sfv, it can find and rename misnamed files
too.


Step 4: get your parity volumes

Download as much Pxx files, as files are missing.
They have to be from the same set. What files you download, doesn't
matter. The index file (PAR) doesn't count of course...


Step 5: recover

Restore your files. You can open the index file or a parity volume
with mirror. So the PAR file isn't really needed.

Missing more files than parity volumes are out there?
Then request a fill.
  

A suggestion about requesting filling:
Let's say, we have 3 parity volumes, but you're missing 4 files: r00 to r03.
You think "Sh**! I need one more file" and request a fill for r00.
Someone other is missing r02 to r05. He requests r05. So the poster needs
to repost r00 and r05.

Better you state _ALL_ missing files in your request:

Attn. John Doe missing foobar.r00 to r03 - need 1 file

Attn. John Doe: missing .r02-05 please post one of these

So the poster will fill r02 (or r03) and it will be enough for both of you...