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Re: Scalable Mail solution with NAS

  • From: Mike Johnson
  • Date: Thu Feb 01 15:52:42 2001

Ron Snyder [snyder@roguewave.com] wrote:
> On Thu, Feb 01, 2001 at 10:12:51AM -0500, Mike Johnson wrote:
> > to deal with concurrent access.  I've yet to find a version of
> > this software that runs on Linux (or any other Open Sourceish OS),
> > so it's not even a consideration for me.
> 
> You mean something like 
> http://www.missioncriticallinux.com/technology/cluster/ ?

Nope, I don't.  See page 4 of their white paper:
http://www.missioncriticallinux.com/technology/cluster/kimberlite.pdf

Quote: 
	Although both systems can access shared disk storage, to ensure data 
	integrity, only one cluster system can run a service and access service
	data at one time. To prevent a service from running on multiple systems
	and corrupting data, each cluster system is remotely connected to the 
	other cluster system's power switch through a serial line. This remote 
	connection enables each cluster system to completely disable the other 
	cluster system by cycling its power. Once a cluster system has been 
	disabled, its services can be safely restarted on the other cluster 
	system.

They go to great lengths to ensure that no more than one system is accessing
a single piece of data at the same time.  Lots of companies do this (maybe
not with this amount of paranoia, but it's still done).  The hard part is
to allow both systems to have access to the same data at the same time.
Write operations on the same file are prevented by distributed lock managers.
This is what Veritas Cluster and Sun Cluster do.  It seems to be relatively
difficult, otherwise I'm sure we'd already see plenty of options.

I looked into the various options (about six months ago) and arrived
at the conclusion that this was best done via NAS boxes.  They handle
locks pretty well, allowing concurrent access to the same file (best
implemented with write locks, allowing read-only access).  This
allows the option of placing a bank of otherwise identical SMTP/IMAP
servers behind a load-balancing switch attached to the NAS.  The
end users only ever see one hostname, but their request would be
handled by the least loaded box.  Need more capacity for more users?
Add more servers.  Need more storage space?  Max out the capacity of
the NAS, then add more NAS boxes.  They're just mount points on the
host.

Mike
-- 
Mike Johnson
Network Engineer / iSun Networks, Inc.
Morrisville, NC
All opinions are mine, not those of my employer





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