[Veritas-bu] too many tapes?

2003-12-02 16:51:19
Subject: [Veritas-bu] too many tapes?
From: alevin AT audible DOT com (Adam Levin)
Date: Tue, 2 Dec 2003 16:51:19 -0500 (EST)
On Tue, 2 Dec 2003, Joost Mulders wrote:
> This is interesting. I looked up the native capacities for all types of DLT.
> Since you say 35GB, I assume you have DLT7000. If not, that may be you problem
> :-) Native capacities are below:
> DLT7000 - 35GB - DLTtape IV

Yes, we're using a DLT7000 drive with DLTtape IV.

> Also: check the density used by the drives. This can be read on the frontpanel
> of the drives if you insert such a tape. It should say "35GB" but the drive 
> can
> be selecting a lower density for various reasons.

Density is correct.

> What I don's grok is that you say 31GB per tape (sounds reasonable) AND that 
> the
> backup already used 80 tapes: 80 x 30 = 2400GB ?

That's exactly what I don't grok, either, which is the problem.  The
backup is filling up these tapes, but I can't figure out what the heck
it's putting on them.  :)

> Also: What is the device file used to write to the tape drives? If you use
> compression on the drive (/dev/rmt/Xcbn), it could be that the compression
> algorithm in the drive chokes on "already compressed audio" and actually
> increases the amount of data although I doubt that this would double it.

Interesting theory.  We are indeed using the cbn devices.  I'll have to
look into this.

> As a last possibility: These tape drives do their best to keep streaming
> ("running"). If you are feeding them not enough data, the drive will start to
> write less tracks on the tape in order to keep running. This will reduce tape
> capacity. If the data rate is falling below a certain (secret I think) limit,
> the drive will do start/stop mode.

This is something I didn't know, and if this is the case, I could
definitely believe this could be the problem.

I think we're currently running only a single stream, for historical
reasons.  However, those reasons don't exist anymore, so I may up the
number of streams, since that'll get more data to the tapes (we have a
library with four drives, two drives per SCSI bus).  That may help if this
is indeed the problem.

Thanks very much.


Adam Levin, Senior Unix Systems Administrator |
Audible, Inc.
Wayne, NJ, 07470                                           "?" he said.

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