> We recently purchased a VTL to backup some short retention backups to.
>  The backups will expire on the VTL and will not be duplicated to tape
> or anywhere else for that matter.  We also have capacity licensing so
> we can create as many virtual tape drives as necessary.  I don't
> believe multiplexing is necessary in a situation like this and that we
> would be better off configuring enough tape drives to handle the
> number of streams we need and turn multiplexing off.  Does anyone know
> of any reason this logic would be flawed...and if not is there any
> documentation out there to support that multiplexing is not a good
> idea when using VTL's?

Good reason #1:  in my VTL testing experience, making a boatload of
drives from the VTL's available disk space leads (of course) to fewer or
smaller tapes.  And a backup that requires a dozen tapes because they're
only 10GB-sized incurs a significant time penalty from all those media
change times.

Good reason #2:  backing the number of drives down produced a need to
multiplex in order to have N jobs in execution (the reason for all those
virtual drives in the first place--throw a drive at every job required).
Multiplexing worked fine.  In non-real-world mux testing (simultaneous
backups of the same directories on a media server), aggregate throughput
improved all the way up to mux 32.

Good reason #3:  the administrative overhead of, say, 128 drives versus
16 quickly became annoying.  Pages and pages of tpconfig listings going
off the screen, or GUI device monitor to look through... hard to see the
drive situation at a glance... process-troubleshooting... it slows
things up in real and (probably) imagined ways.


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