[OpenAFS] AFS and XEN Virtualization

Todd M. Lewis Todd_Lewis@unc.edu
Tue, 10 Jul 2007 07:41:01 -0400

Zach wrote:
> On 7/9/07, Thomas Kula <kula@tproa.net> wrote:
>> AFS does not do read/write replicas.
> Why can't/doesn't it do this? Just curious.

Several possible answers.

(1) It's a _really_ hard problem (with surprisingly little payoff IMHO; 
better to put that effort into buying and maintaining reliable systems and 

(2) It wasn't the interesting problem that AFS was designed to solve. The 
interesting problem AFS was designed to solve is, "How do you support N 
clients accessing the same data when existing systems can only support M 
connections and N is much greater than M?" AFS lets you replicate fairly 
static parts of your file name space across multiple servers so that you 
can (a) support more simultaneous clients than a single server could 
handle, and (b) have the beneficial side effect some degree of redundancy 
in the face of network flakiness.

The fact that this latter feature compensates somewhat when the 
inaccessibility of a read-only server is be due to server trouble and not 
network instabilities is also just a fortunate side effect; it shouldn't 
be misconstrued as a High Availability design feature of the AFS server 
software. In other words, "My servers are unreliable so I'd better run AFS 
on them" isn't a plan for long term happiness.

Other neat things that you get with it (single name space, a real 
authentication system, user manageable and useful ACLs, semi-transparent 
support for multiple architectures in the name space via the nifty "@sys" 
hack, etc.) are icing on the cake, but they fell out of the design process 
(I think; I wasn't there). The point of starting the design, however, was 
to answer the question of N clients on M-connection servers. Read/write 
replication was never on the table.
    / Todd_Lewis@unc.edu  919-445-9302  http://www.unc.edu/~utoddl /
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