On Tue, Nov 06, 2007 at 01:11:04PM -0500, Aleksandr Nepomnyashchiy wrote:
> Can you suggest how I can get more per tape capacity?
> Should I relabel tape using different device (at the moment
> /dev/rmt/0cbn on Solaris 8) ?
> 
> DLTIV tape with DLT7000 tape drive should give me 55-70G

There are two possibilities that I can see:

If your DLTIV media were first used written with an earlier DLT
tape drive like a DLT4000, that tape drive will write a header on
the tape in a non user-accessible location (e.g. the drive firmware
does it) on first insertion.

This header contains the head bias settings used, and the density
the tape was written at. If you later use this tape in a higher-density
drive (like a DLT7000), it will read this header on loading the
tape and set itself to the original density, for compatibility.
(Some drives have density indicator lights and you can actually see
this happening. Drives intended for library operation might not
have them, or they may not be visible..)

An earlier poster alluded to this phenomena.

That header information will never be overwritten by any DLT drive
once written.

And you can't access it via regular commands like mt, dd etc. to
overwrite it either.

If you knew the manufacturer's magic incantation, you could probably
craft an SCB - a SCSI command block - to do it. But there are easier
ways.

In this instance, you need a bulk eraser to destroy the header and
let the DLT7000 write the high-density one.

Unfortunately, you need a very powerful eraser to wipe DLTIV tapes,
as they are both comparatively wide and list amongst the highest
coercitivity tape formats out there. 

You could save much dinero by erasing one side of the tape, flipping
it over etc. - but a one-pass eraser suitable for DLT ran about
$2,000 last I looked, and WILL destroy your watch and credit card
stripes from about 3-4 feet away.

Don't even think about a Radio Shack eraser. Doesn't even come
close. The one you want weighs about 20-30 pounds...

Or:
===

Your data might simply not be very compressible. You generally see
a factor of 2.5 compression (or slightly better), but this really
depends on the data going to the drive.

Text files, web pages - very compressible.

JPEGs, MPEGs, not so much (already compressed).

I've seen Oracle and Sybase backups (of the underlying filesystem
containing the database volumes themselves) that fitted over 100,
120GB onto a DLTIV tape with DLT8000 - lots of zeros in mostly-empty
RDBMS volumes, very, very, compressible.


As other people have said, the backup software simply writes to the
tape until it gets an EOT from the drive, and then re-writes the
failed, last, block to the next tape in sequence. You gain nothing
by re-labelling the tape with the "correct" media type. It won't
help in any way.

I would suggest you proceed by working out which of the two scenarios
is happening. Shouldn't be hard:

i)      Backup 150GB of text files, see how much fits. If the drive is 
        running at high-density, you should get close (or better)
        than rated maximum capacity on the tape.

ii)     Look at the drive indicators, if any. 

iii)    Do a test backup on a scratch tape, record how much fits.
        Send it out for a professional bulk erase pass (about $1-2),
        re-label it and re-run the backup, see if the capacity changed.


HTH.

Cheers,

Andy Sparrow
Consultant.
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