PARs (Parchives) explained.

         

( Modified slightly from an explanation by "rampage", posted to ABWI by "retired" )

The SmartPAR and FSRAID programs generate PARs and recover missing files using mathematical technology similar to that used in RAID systems for the past 20-30 years.

The principle of RAID (R)edundant (A)rray of (I)nexpensive (D)rives

Drive 1 - 25% of data
Drive 2 - 25% of data
Drive 3 - 25% of data
Drive 4 - 25% of data
Drive 5 - Parity data ( equal to the PARs )

A VERY simple example:

Drive 1 data = 01
Drive 2 data = 07
Drive 3 data = 05
Drive 4 data = 04
==
Drive 5 data = 17 (Parity data)


So, if any drive is lost:

Drive 1 data = 01
Drive 2 data = xx <--- bummer
Drive 3 data = 05
Drive 4 data = 04
==
Drive 5 data = 17 (Parity data)

Drive 2 data can be reconstructed by 'simple' math.

01 + X + 05 + 04 = 17
X = 17 - 01 - 05 - 04
X = 07

That is a *very* simple example!

Substitute RARs for the drive data and PARs for the parity data and you've got the principle.

In essence, each single PAR can be used as a 'wild card' for any single missing RAR.

*****

Now, let's plug this tool into everyday Usenet.

Frances wants to post a 10 file set. Being kind, she also uses SmartPAR to generate 2 PARs allowing for 20% file/data redundancy.

The post goes out on a typical Usenet day, and three people download whatever is complete on their servers as follows:

x = got it
o = busted

 

 Larry Moe Curly
r00 x x x
r01 x x x
r02 o o o <--- bummer
r03 o o o <--- bummer
r04 x x x
r05 x x x
r06 x x x
r07 x x x
r08 x x x
rar x x x


Seems like the original news server botched a couple of files at the source.

OK, each person needs the same two fills, and Fran can post either r02 and r03, or post the two PARs, and everyone can complete their download.

On the second day, another 10 part post - same three customers:

 x = got it
 o = busted
 

 Larry Moe Curly
r00 x x x
r01 x x x
r02 o x x <--- bummer
r03 o x x <--- bummer
r04 x o x <--- bummer
r05 x o x <--- bummer
r06 x x o <--- bummer
r07 x x o <--- bummer
r08 x x x
rar x x x

This time, various propagation errors took their toll.

Each person still needs two files, but they're all different files.

Whatever can she do?

She can re-post six files (r02-r03-r04-r05-r06-r07) so that each one gets their two files, or she can post two PARs that each person can use as two wild cards for their two fills!

If you can figure that out for her, you already understand most of the math you're gonna need.

*****

In past practice, adding up all the different requests for missing and incomplete files without PARs has often amounted to a virtual re-post, spanning days (weeks?) of requests and fills.

Today, using PARs, assumptions can be made that the incomplete files and incomplete PARs are distributed somewhat randomly as they rattle around the world; that they amount to something less than 20% of the post, and each downloader will need a few different fills.

If each downloader can get MOST of the complete files and a complete PAR for each missing file, by using SmartPAR, they can reconstruct the complete set as soon as they've finished downloading what they can.

If everyone needed the SAME few files, then posting either fills, or PARs will work.

If everyone needs different files, PARs  are the way to go.


http://www.fluidstudios.com
http://sourceforge.net/projects/parchive
 

Get SmartPAR or Fluid Studios Software RAID Toolkit, and answers to your more technical questions, along with detailed (but relatively simple) instructions on using the program(s).

*****

Suggestions for posting with PARs

Prepare your RARs, etc.

Make an SFV.

Make a set of PARs with about 12% or more, of data redundancy. (The default setting in SmartPAR seems reasonable.)


*****

Downloading with PARs:

Get all the COMPLETE originals you can. Then you won't need any of the PARs.

Get a complete PAR for each missing original. (No! You can't use the same one over and over!)

Run SmartPAR to recreate the missing RARs from the PARs.

If you have SmartPAR installed, you can simply double-click the small .PAR file, and SmartPAR will automatically run and check the archive, recreating any missing RARs if you have the appropriate number of PARs to repair/replace the damaged/missing RARs .


 Original attributed to rampage (pirated and edited for the FAQ without his permission <g>)

 

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