Re: DDS3 Tape drive not compressing?

From: Frank Slootweg (this_at_ddress.is.invalid)
Date: 10/24/05 

Date: 24 Oct 2005 18:36:17 GMT

Adam Skeggs <fun@optusnet.com.au> wrote:
> Hi,
>
> I have a HPUX 11.11 box with a C1537A tape drive which is I believe
> DDSIII. Trouble is I can't see that hardware compresssion is working.

  Check the drive's hardware settings to see if it is not set to no
compression. Most drives can be set to compression, no compression and
software controlled (non) compression.

> Using omniback the media look like DDSIII as the capacity is about 12Gb:
> $omnimm -media_info 581490 -detail
> ...
> Total blocks : 11743616
> (or is this the compressed capacity being reported? No that doesn't make
> much sense)

  It indeed reports the uncompressed capacity.

> Trouble is when I write 12Gb (added up totals from bdf) it's too much -
> thus there is no compression going on it seems.

  bdf info is rather meaningless, because it does not account for the
OmniBack 'overhead'.

  Try to create a datalist/backup_specification which would result in a
*little* more written to the drive, preferably some 13GB. As soon as you
can write more than 12GB, you *know* that hardware compression is
working, because the Native Mode (is uncompressed) capacity of DDS-3 is
12GB, i.e. anything above 12GB is compressing.

  Also make sure that you do *not* specify compression in OmniBack. In
OmniBack, "compression" is *software* compression and software
compression can/will conflict with hardware compression. I.e. if both
are enabled, you will only be able to write *less* than 12GB
(uncompressed), instead of more.

> I am using the compression device: /dev/rmt/c1t0d0BEST which should be
> the right one.

  Make sure that lssf(1M) actually says "best density".

> So I am a bit stumped.
>
> Any clues?
> Thanks,
> Adam.

  If all the above does not help, you could try the ddsinfo utility from
"The Porting and Archive Centre for HP-UX" <http://hpux.connect.org.uk>.
It is rather old and had some limitations. I don't now if there is
anything newer/better somewhere.

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