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