[tahoe-lafs-trac-stream] [tahoe-lafs] #2105: fix the definition of needs-rebalancing

[tahoe-lafs]

#2105: fix the definition of needs-rebalancing
-------------------------------------------------+-------------------------
 Reporter:  daira                                |          Owner:  daira
     Type:  defect                               |         Status:  new
 Priority:  normal                               |      Milestone:
Component:  unknown                              |  undecided
 Keywords:  check verify repair servers-of-      |        Version:  1.10.0
  happiness                                      |  Launchpad Bug:
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 Split from #1784 (also see [ticket:1115#comment:27]):

 The value of {{{needs-rebalancing}}} is computed inconsistently between
 {{{checker.py}}} and {{{filenode.py}}}. In {{{checker.py}}} it is computed
 as:
 {{{
 # The file needs rebalancing if the set of servers that have at least
 # one share is less than the number of uniquely-numbered shares
 # available.
 # TODO: this may be wrong, see ticket #1115 comment:27 and ticket #1784.
 needs_rebalancing = bool(good_share_hosts < len(verifiedshares))
 }}}
 In {{{filenode.py}}} it is computed as
 {{{
 # TODO: this may be wrong, see ticket #1115 comment:27 and ticket #1784.
 needs_rebalancing = bool(len(sm) >= verifycap.total_shares)
 }}}
 where {{{len(sm)}}} is equal to {{{count-shares-good}}}. I don't
 understand this latter definition at all, it looks completely wrong. The
 definition in {{{checker.py}}} is more subtly wrong because it credits
 servers that only have duplicated shares as contributing to existing
 balance. The correct definition should be something like 'iff the
 happiness count is less than the number of uniquely-numbered good shares
 available'.

 I propose to change {{{filenode.py}}} and {{{checker.py}}} to be
 consistent, and to both use the happiness-based definition above.

-- 
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2105>
tahoe-lafs <https://tahoe-lafs.org>
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