[OpenAFS] AFS lag

Ken Hornstein kenh@cmf.nrl.navy.mil
Wed, 27 May 2009 09:33:49 -0400

>>I'm no ubik engineer, but as far as I understand it, the protocol
>>was not designed for even numbers of participating servers. For best
>>results, three or five servers seem to be optimum.
>I hear this frequently, and don't see why it should be true.  The tie
>breaking mechanism during an election is simple.

The tie breaking mechanism isn't really the issue here.

My point is that you gain almost no benefit from an even number of
servers.  Specifically, if you have four ubik servers, you have the
same amount of redundancy as if you have three servers(*); you can lose
one and still maintain quorum.

(*) Okay, purists will point out that this is not exactly true.  If you
have four servers and you happen to lose two, AND one of the two remaining
ones is the "best" server, quorum will still be able to be established.
I could see other reasons for having an even number of servers, but people
should understand exactly what sort of redundancy they can expect out
of a given Ubik configuration.