You can check to see if you're "starving" the tape drives in the bptm logs
on your media server.

If you see "waiting on full buffers", then the data is delivering to the
media server too slowly and your drive is starving.  (Write buffers are
waiting for more client data.  When the buffer is full, it's sent to the
tape drive.)

If you see "waiting on empty buffers", then NB is waiting on the tape drive
to empty the buffer and you're feeding it fast enough.  ("Too Fast" is a
whole different discussion.)
-M

-----Original Message-----
From: Adam Levin [mailto:alevin AT audible DOT com]
Sent: Tuesday, December 02, 2003 2:51 PM
To: Joost Mulders
Cc: Veritas-BU List
Subject: Re: [Veritas-bu] too many tapes?


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

Adam Levin, Senior Unix Systems Administrator | http://www.audible.com/
Audible, Inc.
Wayne, NJ, 07470                                           "?" he said.
973-837-2797     

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