> Martin, Jonathan (Contractor) <JMARTI05 at intersil.com> wrote:
> 
> > There's no way to tell exactly how much more data a single tape will
> > hold, since anything better than 100GB (in your example) is compression
> > and compression varies by file type.
> 
> Yes, I know. I'm not looking for an exact number. I don't know how
> LTO tapes exactly work, but I suppose that the tape is written now
> until a certain "position". If the tape is 609 meter long, it might be
> written up to 550 meters, which would be about 90%. Or is that assumption
> completely off?

That's not exactly how they work, but it's not why the question is hard
to answer.

The main problem is that there is no standard for asking a drive any of
that information.  That's why Netbackup (and every other backup system I
know of) is written so that it doesn't need that information.

I assume that back when a lot of the tape drive standards were being
written that many drives could not detect either the capacity of a
loaded tape or their current position along it.

I do suppose that modern drives keep track of it but don't make that
information available in any standard way.  At least the AIT Mammoth
drives had a utility that would query the drive to display tape usage
and compression average to that point.  I would guess that such
information is available for LTO as well, but I don't know how to ask
the drive for that information.

-- 
Darren Dunham                                           ddunham at taos.com
Senior Technical Consultant         TAOS            http://www.taos.com/
Got some Dr Pepper?                           San Francisco, CA bay area
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