SUMMARY: ufsdump tape unit, st.conf and /dev/rmt/0un vis 0cn

Thanks to all. This list is still the best!

Question:
>Is there any difference between the "/dev/rmt/0un" and "/dev/rmt/0cn"
>tape devices. This is on an Ultra 60 running Sun OS5.8
>
>
>I am trying to get the best from my HP vs80 DLT1 tape drive. This unit
>is an imitation DLT but is supposed to hold 80G. I can only get about
>40G onto the tape before hitting EOT. Using some info from HP's web
>site I added an entry to st.conf. The output from "mt status" seems
>to indicate the change worked. But the backups keep over flowing the
>tape.

Summary:
The first question was driven by the confusing docs in st.conf and
ufsdump etc. The tape drive can be referred to as
	/dev/rmt/0l
	/dev/rmt/0m
	/dev/rmt/0h
	/dev/rmt/0u
	/dev/rmt/0c
five different device names! However the st.conf file only allows
four different settings, which map onto the five different device
names. It is hinted at that "c" and "u" are equivalent but there is
nothing explicit. And given that my problem is "compression" related
this was something I homed in on.

I was using the st.conf as advised on HP's website. The vs80 has no
useful indicators at all. In fact IMHO, avoid vs80's like the plague.
They are not DLT drives at all.

However I ran the backup ten or so times over the week end,
comparing different devices and config file settings. It seems
that everything was working fine. The problem is more an issue of
"lies and damm lies". The advertised compression ratio of 2:1 is a
joke. I have 51GB of data to backup, a portion of which is JPEG
and other imagery. However the vast majority is HTML, pdf, full
text search indexes and other misc content. Backing up to

	/dev/rmt/0mn wrote 38.9G
	/dev/rmt/ocn wrote 46.3G

these figures are based on messing about with the output from
ufsdump, and you never know quite what they mean when they say MB.
Anyway, taking the above at face value, I obtained a compression
ratio of 1.15:1. Useless. I wonder what sort of compression is built
into these drives. Niall O Broin summed things up best:-

At 1:40 PM +0000 22/11/03, Niall O Broin wrote:
>Oh dear - this really is a VVFAQ. This is the second time I've answered
>something like this in a week. The VS80 is a 40GB capacity tape drive.
>From HP's own marketing at
>http://h18000.www1.hp.com/products/quickspecs/11403_div/11403_div.html
>
># Maximum native storage capacity of 40 GB, up to 80 GB assuming 2:1 data
>compression
>
>but unfortunately 80 Gb compressed is pure and simple marketing bullshit. Out
>there in the real world, very little data is 2:1 compressible. At one end of
>the scale, if you have a disk full of empty database files, you may get much
>more than 2:1 compression. At the other end, if you have a disk full of zip
>files, or JPEGs, or MP3s, you'll get no compression at all. The latter is
>rather more common than the former.
>
>I remember years ago doing a little poll here - in the real world, it seemed
>that people were getting 1.4-1.6:1 compression on their mixes of data.
>
>It is unusual that you can only get about 40G onto the tape before hitting EOT
>- your data mix must be particularly unfriendly to the DLT compression, but
>that happens. An alternative explanation is that you are somehow NOT using
>compression, but I think you would see that from the drive's indicator LEDs.
-- 

======================================================================
Fergus McMenemie               Email:fergus@twig.demon.co.uk.
Techmore Ltd                   Phone:(UK) 07721 376021

Unix/Mac/Intranets             Analyst Programmer
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