Re: Third party volume shadowing products?

From: Colin Butcher (colin_DOT.butcher_AT_at_xdelta_DOT.co_DOT.uk)
Date: 07/30/03 

Date: Wed, 30 Jul 2003 18:18:05 GMT

For real high availability I usually get the application to write all data
twice (simply insert your own set of IO routines that the programmers use) -
the locations being pointed to by logical names where one was usually local
disc and the other was a DECdfs remote 'disc'. There's a little fun to be
had with ensuring consistency of data in the data files, but it works very
well indeed. One such system (36+ machines in a control centre environment)
has now been up for just over 14 years with about 20 minutes unintended
downtime (someone's finger slipped about 10 years ago) - and that includes
live upgrades from uVAX II to VAX 4000 to Alpha DS10/20, plus OS version
changes, application changes etc. etc.

DECdfs allows shared read and single write, so it's quite coarse granularity
compared with volume shadowing - however it's far more tolerant of latency
and gives an easy way to make discs available across a network of machines.
Since it uses DECnet underneath it is also very good for high availability
as you can adjust all the DECnet timers for path failover and so on without
disrupting the DECdfs layer - so you don't get 'disc IO' errors.

It's not 'volume shadowing' per se, but DECdfs will allow you to do things
slightly differently and may well achieve a 'better result' depending on
what you're trying to achieve. Worth thinking about. I'm a big fan of it.

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Hope this helps, Colin.
colin DOT butcher AT xdelta DOT co DOT uk
Systems Archaeologist - Investigation & troubleshooting of older systems and
networks.