[OpenAFS] R/W replication

[Nathan Neulinger]

Dirk Heinrichs wrote:
> 
> Derek Atkins wrote:
> >
> > I doubt that RW clones will ever happen.  It would imply huge
> > consistency issues.  Who's job is it to make sure data is consistent
> > across all the servers?  In the current model, data is "pushed" from
> > the RW volume to the RO clones by a sysadmin (or a cron job).  You
> > would need more immediate data consistency with multiple RW volumes.
> > Worse, you also have really hard race conditions.  For example, what
> > if two clients are writing to the same file on different servers?  Who
> > wins?  How do you let the clients know that there was a conflict?
> >
> > Mind if I ask to what purpose you want replicated RW volumes?  You
> > certainly don't need to replicate user's homedirs.  And most system
> > software should be RO anyways (you _do_ realize that you can still
> > mount the RW volume of a replicated RO volume in order to make changes
> > to the volume?)
> I understand that implementing true RW replication has to deal with lots
> of problems.
> 
> The reason I asked was that in case of inavailability of the "master"
> volume, clients cannot save their changes. There could also be another
> solution, that in case the server of the RW volume goes down, one of
> it's RO replicas will automatically be switche to RW, as long as the
> original volume is down.

That logic is fine so long as the volume is truly "down"... But what
happens if it's just inaccessible on the network. A lot of people do
replicas geographically distributed, in which case, that condition will
be relatively common.

-- Nathan

------------------------------------------------------------
Nathan Neulinger                       EMail:  nneul@umr.edu
University of Missouri - Rolla         Phone: (573) 341-4841
CIS - Systems Programming                Fax: (573) 341-4216