[OpenAFS] R/W replication

Derek Atkins warlord@MIT.EDU
14 Feb 2001 09:57:51 -0500

Dirk Heinrichs <heinrichs@qis-systemhaus.de> writes:

> Ok, if it is the link that fails, and not one of the servers, you are
> right. You would need some kind of conflict resolution. I wonder how the
> Coda folks solved this. They support both disconnected operation and rw
> replication. I think they use some kind of transaction processing
> system, similar to  databases. Would be nice to hear from some practical
> experience with this (maybe I'll have to ask on their mailinglist).

Well, OpenAFS should support disconnected operations, once it is
up-ported by the UMich folks (I'm told they are working on it, in
their copious amounts of spare time ;) From memory, disconnected ops
work by saving all "changes" in a local log and then replaying the log
when you "reconnect" to the network.  Because connecting and
disconnecting require user interference (i.e. an fs command), the UI
can work with the user to resolve any conflicts that arrise from the
transaction log playback.

Resolving conflicts in an automatic manner can be, um, challenging.  I
don't know how CODA does it, honestly.  The way I would do it in
OpenAFS, today, is operationally (especially if the issue is having
lots of hosts writing to different files).  Instead of having one
"replicated" RW volume, I would have a bunch of (different) RW
volumes, and have each "file" be in a different volume.  You could
even use RO clones for the other files, and have each "host" know to
mount their particular RW volume and then release the volume whenever
they make changes.

> Bye...
> 	Dirk


       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
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