Re: Disk cache

MooPolice@xxxxxxxxxxxxxx wrote:
>>>Ah, I see... I had not thought of such a scenario. Very interesting.
>>>
>>>So, in a torrent like you described (1 seed, 1 hidden/offline peer, 1
>>>blind peer), assuming downloaders get 1/2 of the file each, they would
>>>be able to finish only if they have complementary halves of the file.
>>>
>>>With a randomization at the piece level, with say, 1,000 pieces in
>>>total, each of the downloaders need the right 500 pieces to make the
>>>torrent healthy.
>>>
>>>The probability for the blind client downloading the right 500 pieces,
>>>not knowing which 500 the hidden peer has, are about 1/(2^1000), since
>>>each piece has 1/2 chance of being the right piece, and that there are
>>>1000 guesses to be made.
>>>
>>>I still think sequential downloading gives the torrent a better chance
>>>for healing:
>>>
>>>Say that each client downloads quarters of the file in random  
>>>order. The
>>>pieces in each quarter are downloaded sequentially. (ie, all the  
>>>pieces
>>>in quarter 1 are downloaded in sequential order, but the file may be
>>>downloaded as Q2,Q4,Q3,Q1)
>>>
>>>In this case, if the torrent behaves like you described above, the
>>>chances for the torrent remaining healthy with the blind and hidden
>>>peers present, but seed absent, are about 1/(2^4), since there are 4
>>>guesses to be made. If each of the clients has 1/2 of the file when  
>>>the
>>>seed goes away, they have a 1/16 chance of finishing.
>>>
>>>So comparing a download strategy of piece-by-piece-randomly (PBPR) or
>>>quarter-by-quarter-randomly (QBQR), the QBQR strategy has a 6.25%  
>>>chance
>>>of finishing, while PBPR has no (1/(2^1000)) chance of finishing.
>>>
>>>I think even the 6.25% probability can be improved:
>>>
>>>If the file is downloaded in halves instead, there is a 25% chance of
>>>finishing this download. Perhaps each client can decide whether to
>>>download halves, quarters, eights or whatever, depending on how many
>>>peers it encounters:
>>>
>>>1 seed, 0 peers  = download halves
>>>1 seed, 1 peer   = download quarters
>>>2 seeds, 2 peers = download eights
>>>
>>>What do you think?
>>
> There is superseeding ... which defeats this appoarch totally.
> 
> MassaRoddel

What does superseeding have to do with improving lifespan of hard drives?

The above strategy seeks to minimize the number of seeks, while not
risking creating a bad torrent. All in all, this thread is about how to
best cache a torrent download without violating other protocol design goals.

Radu



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