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From: chris@mimsy.UUCP
Newsgroups: comp.periphs
Subject: Re: Fast Tape drives
Message-ID: <6583@mimsy.UUCP>
Date: Thu, 7-May-87 02:48:06 EDT
Article-I.D.: mimsy.6583
Posted: Thu May  7 02:48:06 1987
Date-Received: Fri, 8-May-87 05:46:10 EDT
References: <43735@beno.seismo.CSS.GOV>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 56

In article <43735@beno.seismo.CSS.GOV> mo@seismo.CSS.GOV (Mike O'Dell) writes:
>In a recent posting here Chris Torek posted some calculations showing
>that 125 ips 6250 tape drives were "unnecessary" since you couldn't
>drive them full speed.

>HORSE HOCKEY!!!

>The problem with his calculations is that UNIX doesn't do overlapped
>I/O on raw devices, tape or disk!

*Who* does not?

I should have mentioned this.  (Indeed, I was going to do so; I
cannot now recall why I did not.  Perhaps I simply forgot.)  4.3BSD
dump uses multiple processes; I use kernel hacks.  Either way, we
get overlapped I/O.  Put the 4.3 dump on your Suns if Sun has not
done it for you yet in whatever you are running: it makes an enormous
difference.

If you use tape for other things, without the asynchronous hacks,
yes, tape speed is important.

>I have a Fujitsu streamer on my SUN 3/280, and it streams some
>large part of the time doing dumps, but when it stops, backs up,
>and has to get a running start again, it takes forever by comparison.

Streaming drives are killers.  Fortunately, a Sun 3/280 with Eagles
is a lot faster than a Vax 11/750 with RA81s.

>Further, if you every use the tapedrive for anything besides
>dumps, like really writing tapes full of data, the speed of the
>drive matters even more [...] .

(Unless you can convince them to speak asynchronously.)

>Nothing beats a REAL tapedrive, i.e., 1600/6250 bpi @ 125 ips
>start/stop, ideally with 200 ips rewind and fast-forward speeds.

This is `real'?

>Of course these specs are a compromise - until you see 250ips @
>6250 gcr mode, you ain't seen a tape drive.

Now *this* is more like it!  Unfortunately, these drives come with
IBM machines and COBOL programmers. :-)

Incidentally, if you do image backups:

	dd if=/dev/rhp4a of=/dev/rmt8 bs=32k

(or whatever) from Eagles or faster, you *can* run 125 ips at 6250
bpi.  Alas, you then need a spare drive to recover files, unless
you like writing *big* programs....
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris