In terms of the read any/ write all strategy thats
exactly what I want.  I do want it to deliver ail to
all the servers at once.  I want a system exactly as
yuo said, so that if one location completely loses
internet connectivity, users outside that location can
still access the email.  Coda seems to be theonly
system I know about that has read/write replication. 
Even AFS only supports read/only replication.


--- Jan Harkes <jaharkes_at_cs.cmu.edu> wrote:
> On Wed, Oct 10, 2001 at 10:02:19PM -0700, Zachary
> Denison wrote:
> > drive, where I store users mail directories.  What
> I
> > would like to do is setup these three machines as
> CODA
> > servers with replication, so each one is an exact
> > mirror of the other 2.  Is this what happens under
> > replication?  I hope so, because this is what I
> want. 
> > Anyway I want to store these in geographically
> > disparate locations.  At each location I want to
> have
> > CODA clients, which run the mail delivery
> software.
> 
> Won't work.
> 
> Coda uses a write-all, read-one replication
> stragegy. So the Coda
> clients in each location will try to store new mail
> in all servers at
> the same time.
> 
> So you could just as well have all servers in one
> location, in which
> case you probably wouldn't consider delivering email
> from a remote
> location. This just shows that Coda is not the right
> solution for your
> problem.
> 
> > the administration manual, it seems to imply that
> the
> > maximum size of the RVM log partition is 130 Megs,
> 
> RVM is both a log and a data partition, the log
> really doesn't have to
> be that big at all, somewhere between 2 and 8 MB is
> plenty. It's the
> data segment that is important, this is memory
> mapped, so the limit is
> pretty much defined by the operating system and is
> typically between 2
> and 3 GB.
> 
> The RVM data contains all the metadata and directory
> contents, which is
> typically around 5% of the actual size of the
> associated filedata. So
> there is a limit to how much data (actually how many
> files) a server
> process can possibly handle, which we _assume_ to be
> around 50GB.
> Ofcourse it is possible to some extent to run
> multiple Coda server
> processes on one machine, but that is
> administratively more difficult
> and you'd be living out of swap, as 10 processes
> would easily need about
> 20GB of swapspace. This kind of VM load is typically
> not handled very
> well, so you'll not only get hit by obscure Coda
> bugs, but also obscure
> Linux VM bugs ;)
> 
> So why do you want replication? To ease the load of
> many users popping
> their email, or to provide failover when one of the
> sites disappears
> from the face of the earth?
> 
> For the user case it should be possible to use a
> static load-balancing
> trick where each server only deals with a 1/3rd
> subset of all users. If
> the group is large enough each server should be
> getting a similar number
> of users at any given time. Each mail delivery
> process can accept
> anything, but forward specific users to the right
> server, f.i. depending
> on the md5 hash of the username % 3.
> 
> For failover, it just depends how synchronized it
> need to be. Any good
> synchronization will require a lot of traffic
> between the 3 servers.
> Effectively the total user load such a setup can
> handle would be less
> than any single machine would be able to.
> 
> Jan
> 


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