[tahoe-dev] "servers of happiness" Re: MapReduce over Tahoe-LAFS slides from HadoopWorld

Zooko Wilcox-O'Hearn zooko at zooko.com
Tue Oct 13 13:23:49 UTC 2009

I asked my wife Amber for help formalizing my intuition about what  
sort of share placement makes me happy.  We came up with this:

First of all, let's call a set of servers "sufficient" if you can  
download the file from that set (i.e. if at least K distinct shares  
are hosted in that set of servers).

Now consider the largest set of servers such that every K-sized  
subset of it is sufficient.

Let's call the size of that largest set S.  Now my intuition about  
"Happyness" is that I configure a Happyness number H, and if an  
upload results in an S >= H then I'm happy.

I think this is also Robert Metcalf's intuition [1].  It may also be  
Shawn Willden's intuition [2], but on the other hand perhaps Shawn  
Willden's intuition is something more sophisticated. ;-)

A neat thing about this way of thinking is that the number S is the  
"health" or "robustness" of the file.  An upload or a file-check  
operation could report S to the user.

What do you think -- is this measure of "health" a good enough  
measure for the purposes of ticket #778?



[1] http://allmydata.org/pipermail/tahoe-dev/2009-August/002494.html
[2] http://allmydata.org/pipermail/tahoe-dev/2009-October/002972.html

http://allmydata.org/trac/tahoe/ticket/778 # "shares of happiness" is  
the wrong measure; "servers of happiness" is better

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