Newsgroups: comp.periphs.scsi
Path: lth.se!sunic!mcsun!uunet!kentrox!ronc
From: ronc@kentrox.uucp (Ron Christian)
Subject: Re: EXB-8200 transfer rate
Message-ID: <1992Aug24.182613.2016@kentrox.uucp>
Organization: ADC Kentrox, Portland OR
References: <1992Aug19.212240.6537@scf.loral.com>
Date: Mon, 24 Aug 1992 18:26:13 GMT
Lines: 43

In article <1992Aug19.212240.6537@scf.loral.com> andres@scf16.scf.loral.com (Jeff Andres) writes:
>
>Hi! I'm posting this questionfor some fellow engineers who are having
>problems in determining the length of a 8mm tape in bytes. They are
>using a CMS tape drive EXB-8200. They wrote a program to write 512 Kbyte
>blocks and it has written to the tape drive for nearly 24 hours and is
>still going...This doesn't make sense that an 8mm tape can hold this
>much information.
>
>Can anybody out there assist us? Does anybody out there have experience
>with the tape drive ? and can you write larger than 512K blocks ?

Do you mean 512 K bytes, or 512 bytes?  If the former, I don't know what
might be the problem.  If the latter, you have stumbled on a common weakness
of the 8mm drive.

The 8mm drive needs to write in 1K byte blocks or larger, due to inaccuracies
in the head positioning hardware.  (The heart of the 8mm drive is only a
consumer video device, after all.)  Using too small of a block size has the
following effects:

(1)  Write speed suffers dramatically due to increased amount of head
repositioning.  (This may be what you're seeing.)  In general write speed
to any streaming device will suffer with small (512 byte) blocks, but on
the 8mm the slow-down is incredible.

(2)  Head wear increases dramatically for the same reason.  Drive failures
will be much more frequent with small block sizes.

(3)  If the info you put on the tape (a label, for instance) is less than
512 bytes (only one block is written between file marks), subsequent tape
operations (a Unix mt command, for instance) may or may not find the
file on the tape.

These limitations are not well documented.

Try increasing the block size and see if that fixes the problem.


			Ron
-- 

		Barbara Bush - Colin Powell in '92
