I am following up to address some of the concerns raised by several
people regarding the quorum issue in AFS 3.4. Essentially, I had my
facts incorrect throughout this discussion, as was pointed out to me by
a couple of people in Transarc.
Marc Horowitz <[EMAIL PROTECTED]> writes:
| >> Well, if the problem is technically hard to solve, it doesn't become any
| >> easier just because it is on everybody's wish list. Look at how much of
| >> effort has been put by the database vendors like Sybase and Oracle into
| >> replication, with not much practical success. And yet it is the
| >> number-one feature wanted of any RDBMS today.
|
| The database replication problem is a harder problem than what we are
| asking you to solve with AFS.
|
| Could you explain, in detail for those of us with extensive experience
| with afs internals, why a vlserver which loses quorum should not
| continue to allow lookup requests on its database?
I was thinking of the one-cell-across-sixty-cities problem, rather
than the problem of simply read-only information without quorum, when
I wrote that; I suppose the quorum issues are a subset of the larger
issue of disconnected database operations.
|
| The only counterargument I can think of is that if you have weird
| asymmetric routing, then a client with access to the vldb sync site
| can perform vldb operations which affect cache managers which cannot
| reach a vlserver with quorum, causing the client to see old data or
| have a volume "disappear". I think this is a worthwhile risk.
|
| Marc
You are correct; I have had it wrong throughout this thread. As a
matter of fact (which it wasnt earlier), as someone in Transarc
pointed out to me, the vlservers have been satifying read-only
transactions since AFS 3.2.
Therefore, coming back to the question that [EMAIL PROTECTED]
asked:
"If my AFS server (running all the servers) is cut-off from the rest
of the servers and goes out of quorum, will the clients connected to
this server still be able to access volumes that are on it?"
The answer is a yes, from 3.4 onwards, since the ptserver was modified
to allow r/o access outside quorum too. The 3.4 beta currently out
should work this way. Of course, the assumption is that the top levels
of the AFS tree are replicated on this cut-off server, so that one can
actually get to the more interesting volumes.
