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

[tahoe-lafs]

#778: "shares of happiness" is the wrong measure; "servers of happiness" is
better
--------------------------------+-------------------------------------------
 Reporter:  zooko               |           Owner:  zooko   
     Type:  defect              |          Status:  assigned
 Priority:  critical            |       Milestone:  1.6.0   
Component:  code-peerselection  |         Version:  1.4.1   
 Keywords:  reliability review  |   Launchpad_bug:          
--------------------------------+-------------------------------------------

Comment(by kevan):

 I agree with David-Sarah on the idea of having two error messages. For the
 error message where there are fewer servers than k, it now prints
 something like:

 {{{
 shares could only be placed or found on 2 server(s). We were asked to
 place shares on at least 4 servers such that any 3 of them have enough
 shares to recover the file.
 }}}

 and, in the other case, it prints something like:

 {{{
 shares could only be placed on 5 server(s) such that any 3 of them have
 enough shares to recover the file, but we were asked to use at least 7
 such servers.
 }}}

 Note that this particular error message is only output if the peer
 selection process managed to place all of the shares -- this means that
 there will always be at least one server, so we don't need to worry about
 special-casing it for 0 servers. In cases where the selection process
 wasn't able to place all of the shares, the error message looks like:

 {{{
 peer selection failed for <Tahoe2PeerSelector for upload dglev>: placed 0
 shares out of 10 total (10 homeless), want to place on 4 servers, sent 5
 queries to 5 peers, 0 queries placed some shares, 5 placed none (of which
 5 placed none due to the server being full and 0 placed none due to an
 error)
 }}}

 Here, servers_of_happiness is handled by "want to place on 4 servers";
 this is a necessary condition for happy=4 to be satisfied, but I think it
 could probably be improved, so I'll look into doing that if no one
 disagrees.

-- 
Ticket URL: <http://allmydata.org/trac/tahoe/ticket/778#comment:92>
tahoe-lafs <http://allmydata.org>
secure decentralized file storage grid