[tahoe-lafs-trac-stream] [tahoe-lafs] #2107: don't place shares on servers that already have shares

[tahoe-lafs]

#2107: don't place shares on servers that already have shares
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     Reporter:  zooko            |      Owner:
         Type:  enhancement      |     Status:  new
     Priority:  normal           |  Milestone:  undecided
    Component:  code-            |    Version:  1.10.0
  peerselection                  |   Keywords:  upload servers-of-happiness
   Resolution:                   |
Launchpad Bug:                   |
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Comment (by daira):

 Replying to [comment:4 zooko]:
 > [...] Here are some possible rules:

 [added numbering for ease of reference]

 > 1. [bad rule]: Don't put a share on a server if that server already has
 a different share. Sometimes, as Daira points out, we would have to break
 that rule in order to achieve Servers-of-Happiness.
 >
 > 2. [bad rule]: Don't put a share on a server if that share is already on
 another server. The same example Daira gave makes it so we would have to
 break this rule in order to achieve Servers-of-Happiness.
 >
 > 3. [good rule?]: Don't put a share on a server unless doing so would
 increase the Servers-of-Happiness-Level.
 >
 > 4. [good rule?]: Don't put a share on a server unless doing so is a
 necessary placement in an upload placement that would reach the required
 Servers-of-Happiness-Level {{{H}}}.

 I'm confused as to why we're trying to redesign this now. Doesn't the
 algorithm based on Kevan's thesis that Mark has already implemented
 (modulo some minor details) already satisfy 3?

 Rule 4 seems ambiguous if "necessary" means "necessary to achieve
 happiness". The problem is that there can be multiple ways to achieve
 happiness using different subsets of a given share placement, and none of
 these may be "necessary" given that the others exist.

-- 
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2107#comment:6>
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