(Illustration by Gaich Muramatsu)
On Tue, Oct 05, 1999 at 11:44:08AM +0200, Gulcu Ceki wrote: > The third paragragh of that section reads: "If you already have a > read-only replicated root volume but want to update it, you should > mount the read-write version of the root volume elsewhere and make > changes to this volume." ... > Clearly, it is unwieldy to have the administrator intervene manually > whenever "updates" are made to the root volume. That is illogical > since it would mean that Coda is more cumbersome than tools like > mirror and rdist. > > I am surely missing something. Can you enlighten me? Hi, Read-only replication is not used that much anymore. The reasons for read-only replication are still the same. Conflicting updates cannot be allowed to occur at the root volume since this would make the entire Coda file system inaccessible. However, if the root volume is not replicated, the availability of the entire Coda file system depends upon the availability of the server acting as the custodian for this one volume. But instead of having difficult to maintain read-only replicas, it is simpler to have a regular read-write replicated volume and then use ACL's to protect it from updates by users who are not administrators, this ofcourse assumes that sysads know not to mess with the root volume during working hours ;) The advantage of rw-replicated volumes are; - Clients can cache the file objects in the volume, and perform rapid cache validation when reconnecting. With non-replicated or read-only volumes there seem to be some problems, their objects tend to be ejected from the cache quite soon after disconnection, which reduces the usefulness of disconnected operation. - The replication strategies will make sure that all replicas are consistent. So the administrator does not have to use the complicated dump/restore sequence. - ACLs protect the volume from possibly conflicting updates by regular users. A disadvantage is, that it is possible to get a very difficult to repair conflict in /coda (the root directory of the root volume). The only way to fix this is by creating a temporary root to mount and repair the original root volume. The `alert' sysadmin will normally make sure that all servers are up, that his client is strongly connected to all servers, and that he is the only person modifying the rootvolume at that time, to avoid such a conflict. JanReceived on 1999-10-05 10:44:08