Coda File System

Replication (was Re: High availability)

From: Bill Carlson <>
Date: Wed, 22 Sep 1999 16:29:59 -0500 (CDT)
On Wed, 22 Sep 1999, Jan Harkes wrote:

> The only way in which the SCM is special, is that it is the server where
> all others are fetching updated volume location and user databases from.
> So the SCM is only special in the cases where new users/volumes are
> added. And by replacing the name of the SCM in /vice/db/scm, and
> restarting the update daemons on all servers it is possible to switch to
> a completely different SCM.
> As all servers have their own local copy of the volume location and user
> databases, no server relies on the SCM during normal operation, and
> clients can authenticate and find volumes in the cell by asking _any_
> available server. Point a client to a single server in the cell, and it
> will be able to find all others.

What affect would there be if the SCM died? Assume total loss of the
machine is question, no hope of recovery.

> Replication in coda works as follows:
> - When a client writes to a replicated volume the update is sent to all
>   accessible servers in the replication group.
> - When a file's attributes are fetched, the version vectors from all
>   accessible replica's are collected and compared. If there is a
>   difference, some server has missed updates, and a server-server
>   resolution is triggered.
> - The file is fetched from any of the available replicas.

It seems to me there has recently been an interest in a distributed file
system that is not server centric and further more would all a single
volume space to be made up of multilple machines such that the total
volume space avaliable is close to the sum of the space. Something like
RAID5, where a single machine lost would not result in data lose nor would
the filesystem be inaccessible, yet the cost in terms of redundant
machines would be small. Coda is already there in terms of mirroring,
would a more general approach be in the future?

Bill Carlson
Systems Programmer	|  Opinions are mine,
Virtual Hospital        |  not my employer's.
University of Iowa Hospitals and Clinics	|
Received on 1999-09-22 17:32:33