> This has been an interesting discussion for me and I have some further
> comments. Here are the two systems I see as being advocated and why each
> has severe shortcomings.
>
> The NFS solution
> Centralized storage (a NetApp in our case) which is shared via NFS to a
> pool of IMAP servers behind a Foundry ServerIron doing a nice job of
> load balancing users. Clients connect and are sent to whichever IMAP
Is there any data on the extent to which imapd needs to be load balanced?
Is it just because of all the fork()ing?
> server is least busy and each server accesses files via NFS. The pluses
> for this approach are a robust storage system with alerts, double parity
> and enterprise quality. Shared IMAP servers so no user is ever down if a
> server breaks, the load balancer sends the user to another machine and
They will be if the storage server breaks. Is that significantly less
likely than an IMAP server breaking?
> The Direct Attached method
> What I have heard (or misunderstood from all the discussion) is that
> Mark advocates lots of servers with direct storage. In this situation I
> would probably take my NetApp and use either a SAN (if I had a budget)
> or an iSCSI connection and carve out block storage to each IMAP server.
> Each server would then host a group of end user accounts. I would then
> have to either give different groups of end users different FQDN's to
> attach to the service or use something like Perdition (alternatives?) to
> route users from a single FQDN (like imap.uci.edu) to the proper server.
> I might even use a load balancer to front a couple of Perdition machines
> for redundancy. The pluses would be that I could use far more efficient
> and scalable back end mail formats (mix/mbx) and I would eliminate all
> of my performance (fingers crossed) problems. It would eliminate that 1
> in 10,000 users who have a multiple client access issue. The minuses are
> that I need a couple more machines (not the end of the world) and that a
I'm not convinced that you would. The system you've just described is
simpler than the NFS-backed setup, and may very well require less computing
resources.
> single IMAP server failure will result in a service outage for some of
> my end users. That's not acceptable and is a big problem for management
As I asked above; is there reason to believe an IMAP server failure is
more likely than a storage server failure?
> as well as it should be I might add. I also need to guess correctly and
> distribute my users to various machines and balance them manually
> instead of letting the load balancer do it dynamically and best of all,
> without needing my intervention.
That comes back to what is really responsible for the load. Any data?
Cheers,
- Joel
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