Coda File System

RE: BeOS & Coda server replication

From: Peter J. Braam <>
Date: Sat, 5 Jun 1999 12:45:46 -0600
There is a bit more to replication than is said here.

AFS and DCE/DFS have "replication" which is merely like a "cron" job.  Every
so often the read only replicas are updated.

Coda has true real time replication servers, and additionally high
availability by allowing updates in partitioned groups of servers.  If
diverging replicaas arise due to server or network outages, then Coda's
resolution mechanism will generally fix them up (unless they are true

The basic mechanism here is as follows:
- client gets version stamp from all servers carrying an object
- if these agree, file or directory is read from a randomly chosen server
- if these differ, client instructs the server to resolve differences.

Coda needs very extensive versioning information to do this, which is not
present in AFS or DCE/DFS metadata - one needs to be able to detect that
certain servers have missed updates, i.e. there is a partial ordering on the
versions of files, which is implemented with version vectors.  It is also my
impression that the Windows 2000 and NT directory replication are more like
that of AFS, and not like Coda.

There is a similar issue going on in reintegration after disconnected
operation.  This can be done with AFS as both Transarc and ARLA have
demonstrated.  But AFS lacks versioning information which allows Coda to
easily resume after a failed reintegration - unlike the server replication
where AFS is really lacking meta data, this one isn't too serious probably.

- Peter -

On Sat, 5 Jun 1999, _Oliver,.Thuns: wrote:

> >It also supports replicated servers, which are
> >aimed at providing higher availability to clients.
> You could also replicate servers with AFS, but only read-only (and one
> read-write). Could coda provide replicated servers with write access?

yes, u could. coda assumes that write-write conflicts are rare(mostly
true) and proceeds, in case there are conflicts u will have to manually
resolve the confilcts.

radha krishna
Received on 1999-06-05 14:43:46