[TriLUG] clustering or server mirroring

[Mike Johnson]

Aaron S. Joyner wrote:

[snipping stuff more relevant to entire discussion to weed out one block
  to make a particular point]

> Consider something other than web services.  Something like Cyrus IMAP, 
> for example, is notoriously difficult to setup for high availability.  
> The traditional way to handle the problem is to use one big box, make it 
> as redundant as possible, and hope it doesn't go down.  Anything more 
> complicated involved a "murder" of IMAP servers (I'm not making that 
> name up), with a specially designed redirector, and a lot more 
> machines.  Essentially, this becomes a configuration and management 
> nightmare, unless you *really* need to scale well beyond what one box 
> can serve up anyway.  With Cyrus, you can't even share the backed 
> storage over NFS, because it's locking doesn't play well with NFS.  So, 
> how do you provide good email services?  Enter DRBD.  

[snipping detailed description of DRBD and how it works]

Um, wow.  You have to do all that [DRBD] to fail-over Cyrus?  Ick.  This 
is why maildir is so nice.  Between IMAP/POP and SMTP, it's actually why 
maildir was created.  Keep your spools on an NFS system and you can have 
multiple IMAP servers with simply an IP level load balancer and you're 
set.  One of the IMAP servers dies?  No big deal.  The same can be 
said/done with SMTP.  Both can easily scale to multiple systems.  This 
relies on a reliable NFS system, but those aren't too expensive.

> There is of course the classic way of clustering, involving external 
> shared storage.  It's very similar to the DRBD description above (in 
> fact the story should be told in reverse, as DRBD is based off this 
> idea), where you have an external source of shared storage, such as a 
> SAN fabric, or a shared SCSI disc subsystem.  Instead of having a shared 
> Gigabit link to mirror the data back and forth, it's simply stored on 
> media which both machines have access to.  The obvious problem here 
> being that Brocade switches and fiberchannel disc controllers / disc 
> boxes aren't cheap equipment.  :)  Even external SCSI arrays are usually 
> a bit of overkill for the task at hand, not to mention equally expensive.

Everyone remember Oracle's talk on using a firewire disk for shared 
storage?  I'm not sure where they are on filesystem support, but it's 
worth looking into.
http://oss.oracle.com/projects/firewire/

On DRBD, what happens if the gigabit link between the systems fails? 
Does it scrag your filesystem?

Mike