[OpenAFS] no quorum elected

[Stephan Wonczak]

   Hi Robert!

On Mon, 2 Jun 2008, Robert Banz wrote:

>
> On Jun 2, 2008, at 11:30 PM, TIARA System Man wrote:
>
>> thank you russ.. i just check my CellServDB files on each file server. i 
>> just found one has wrong db info in the file. :$
>> 
>
> it's generally good to have at least three DB servers (an odd number is 
> important!). The two most common causes of the quorum error are not having a 
> majority of the DB servers available, or, having a time split between them.
>
> File it away for future reference!

   This, of course, is wrong in the case of AFS DB-Servers. The 
master-server (usually the one with the lowest IP) has an additional 
half-vote. So no split-brain possible here.
   If you have 2 servers, and connection is severed, you have 1.5 votes on 
one side, and 1 on the other. Since the cluster knows there are supposed 
to be 2.5 votes in total, the single slave server will refuse to accept 
changes (while happily continuing to serve requests with older data)
   In the case of 4 servers (which we have in Cologne) you will get the 
exact same scenarion, only with 2.5 to 2.0 votes. If only the master 
server is isolated you get 1.5 to 3.0 votes. This will result in the three 
still connected servers voting for a new master, and the old master will 
stop accepting changes since it knows that it is in the minority in this 
situation.

   Cheers from Cologne,

 	Dipl. Chem. Dr. Stephan Wonczak

         Zentrum fuer Angewandte Informatik (ZAIK)
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