Subject RE: [firebird-support] Re: Shadow files on a network share
Author Alan McDonald
> > - Shadowing can occur only to a local disk.
> This is not our experience...!

Then what is your experience - cause it has never worked off a system
drive - i.e on a share. perhaps a local mount is OK but never on another PC
across a network...

> > In any case, attempting to maintain a valid shadow over a network
> > connection to a remote box is chancy at best. I don't know this
> for
> > certain, but I'd expect that it would be relatively easy to corrupt
> > the shadow in case of network difficulties without receiving any
> > errors back to the server. Then, something bad happens to the
> > primary database, you try to flip over to the shadow, and discover
> > that it is virtual gibberish.
> This is not our experience from testing - with a "manual" shadow you
> prevent additional connections if a shadow goes away. Obviously this
> represents an additional point of failure, but out problem is not so
> much 100% availability as 100% data recovery at any moment in time. So
> far in testing all we seem to be able to lose are any transactions
> active at the moment of failure.

what's "manual" shadow when it's not a shadow?
and you may wish to do some more testing - unfortunately, sooner or later,
you may get a corruption and shadowing will also get it.

> Our problems would be solved if we could immediately activate a shadow
> on a backup server in times of need. We have already looked at
> replication but we need something more "real time", and have already
> considered mirrored disks but will not necessarily have timely
> physical access to the hardware.

Maybe you should look at the logging mechanisms offered by IBLogManager -
there is soon to be roll-ahead logs as well as roll back.