[tahoe-dev] [tahoe-lafs] #778: "shares of happiness" is the wrong measure; "servers of happiness" is better

Shawn Willden shawn at willden.org
Sun Aug 23 06:09:07 UTC 2009

On Saturday 22 August 2009 01:41:22 pm tahoe-lafs wrote:
>  I imagine that Shawn's efforts will result in a
>  web page on the client node which has knobs for encoding parameters and
>  displays probable reliability properties as an output, to help users make
>  those decisions. Or the other way around, where the user dials in the
>  reliability requirements, and is informed of the encoding strategy (and
>  resulting expansion factor).

The latter is my plan, except without describing the encoding strategy.

My thinking is that the k-of-N encoding strategy is not very meaningful to 
people, especially when shares may not be perfectly distributed.  The k-of-N 
server strategy is a little better, but has the same basic problems.  Of 
course, small probabilities are also not very meaningful, but I think they're 
better, especially if scaled up with significant numbers of files over 
significant periods of time.  I also think it's probably more useful to 
describe expansion factors in terms of amount of data that can be backed up, 
assuming fairness and their current available storage.

So, I want to provide users the ability to choose between a few different 
reliabilities, expressed as probability of losing a file per year (or maybe 
per decade), each coupled to a volume of data that can be backed up, and 
perhaps also a backup time estimate.  I think that puts the choice in terms 
that most people can relate to.


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