Veritas-bu

[Veritas-bu] Howto check the approx. available space on a tape?

2007-03-29 11:45:27
Subject: [Veritas-bu] Howto check the approx. available space on a tape?
From: austin.murphy at gmail.com (Austin Murphy)
Date: Thu, 29 Mar 2007 11:45:27 -0400
On 3/29/07, Alexander Skwar <listen at alexander.skwar.name> wrote:
> Martin, Jonathan (Contractor) <JMARTI05 at intersil.com> wrote:
> > There's no way to tell exactly how much more data a single tape will
> > hold,
> ...
> 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?

You can't make that assumption.    LTO1 and 2 write 8 tracks at a time
(LTO3 writes 16),  along the entire length of the tape. When it gets
to the end, it shifts the heads a bit and does the same in reverse.
An LTO-1 tape has 384 tracks in total. That means the 8-track head
makes 48 passes to fill it up.

It reads the tape as it writes and if there is an error, it leaves the
bad block in place and rewites the data on the next tape block.  BOT,
EOT, IRG, records, blocks, etc. are all logical.  I don't know of any
way to tell what % of the expected tape capacity was used to record a
given amount of data.

If you write un-compressible data (or turn off compression) and have a
good tape and tape drive, you should get the stated capacity to within
a couple of gigabytes.   I frequently get ~196 GB on my LTO-2 tapes.
If you have a tape with a high error rate, the "full" capacity could
be substantially less.

Austin