[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:           |     Status:  new
  enhancement            |  Milestone:  undecided
     Priority:  normal   |    Version:  1.10.0
    Component:  code-    |   Keywords:  upload servers-of-happiness brians-
  peerselection          |  opinion-needed
   Resolution:           |
Launchpad Bug:           |
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Comment (by zooko):

 Replying to [comment:2 daira]:
 > I think you're misinterpreting the thesis. It says not to put a given
 share on the *same* server if it already exists there; not that upload or
 repair can't create shares that are duplicates of existing shares but on
 *different* servers that already have (other) shares. The latter is
 clearly necessary to achieve happiness. (Proof: suppose that one server
 has all shnums, the remaining servers have shnum 0, and shares.happy > 2.)

 Good point. Here are some possible rule:

 • [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.

 • [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.

 • [good rule?]: Don't put a share on a server unless doing so would
 increase the Servers-of-Happiness-Level.

 • [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}}}.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2107#comment:4>
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