Re: [Dibs-discussion] Congrats & some problems

[Emin M. <em...@al...>]

Christian Stork <dib...@cs...> writes:

>> > In the above example of 10 peers with redundancy 2 and kbPerFile = 1MB
>> > the result would be that all files smaller than 1MB are "redundanized"
>> > (better word?) to three equal-sized copies and these three copies/pieces
>> > are randomly distributed on different hosts.  Furthermore, each of these
>> > copies suffices to reconstruct the file.  Is this correct?
>> 
>> Yes.
>
> What's your benefit of using Solomon-Reed then?  Three regular copies
> would do, wouldn't they?

In this case, there is no benefit of using Reed-Solomon (RS) codes,
and the pieces files are actually copies of each other.  Then benefit
of RS codes come in when you split files.  

For example, imagine that your kbPerFile limit is 1MB and you want to
backup a 10MB file such that you are robust to 2 peer failures.  When
set to produce 2 redundant copies, DIBS will split the file into 10
pieces, produce 2 redundant copies and spread these among different
peers (assuming you have enough room on enough peers).  The total
amount of data backed up is 12 MB (plus a little overhead for
encryption).

If you had used straight copying instead of RS codes and you wanted to
be robust against two peer failures, you would need to produce 3
copies for a total of 30 MB and split these 3 copies amongst the 12
peers.  To summarize, the benefit of RS codes comes into play once
files are split into more than 1 piece.

-Emin