[OpenAFS] R/W replication
Wed, 14 Feb 2001 07:12:54 -0600
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 Neulinger EMail: email@example.com
University of Missouri - Rolla Phone: (573) 341-4841
CIS - Systems Programming Fax: (573) 341-4216