Data Redundancy made simple

[Jason Pascucci]

In article <992 at bcstec.boeing.com>, ruben at bcstec.boeing.com (Reuben
Wachtfogel) writes:
|> I could store dual copies of these files on 2 seperate nodes and 
|> code the application to perform all updates of
|> these files on this MASTER node as well as a BACKUP MASTER node
|> so that if the MASTER were vaporized, the BACKUP could be slipped
|> in with minimal downtime.
|> 
|> My question is: 
|> 	
|>          'Is there some application transparent way to achieve
|>           data redundancy in a unix network ?'
|> 

I haven't heard about anything which would do this for you,
but here are a few ideas which might solve your problem:

NFS helps a great deal. I don't know if your current filesystem
scheme will do this, but when you make an NFS request to a site which is
down, it will hang, not crash. This is a big plus from your point of
view.
(I personally hate this, esp. with my Sun. Ick)

Now, there are a few things you could do. How about setting
up a 'Secondary' system which has a process that constantly checks to
make 
sure the 'Master' is alive. If it isn't, have it restart TCP/IP with
the address of the 'Master', and NFS will resume talking with you
instead of him. When the Master boots back up, have it copy over the 
current data, and Re-Shutdown the Slave, and have the Master take over 
yet again. Admittedly, it's not elegant, but it should do what you want
with a minimum of work.

If you want to throw a little hardware at this, You could combine the
Network stuff with Dual Ported SCSI, assuming the OS supports Priority 
Select. This would allow you to use extremly up-to-date data, and is a 
large win. This doesn't guard against disk failures, though. Another 
solution is to have backups on the 'Secondary' system, and still
do the dual porting. You may also want to look into Third
party vendor (I don't think HP's unixii do this, but I don't claim
to know for sure) who do disk mirroring, and Dual port one half
(Or both) of the mirror. There are all sorts of Hardware 
you can throw at the problem, if you want to spend the money.

Your Milage May vary. These are, of course, only suggestions.

--
Jason R. Pascucci           "Kate Bush Is God!......Oops. Wrong
newsgroup"
jasonp at primerd.prime.com

Disclaimer: My company isn't responsible.