From: ronc@kentrox.uucp (Ron Christian)
Subject: Re: EXB-8200 transfer rate
Organization: ADC Kentrox, Portland OR
Date: Fri, 28 Aug 1992 16:46:24 GMT

In article <1992Aug27.172541.136251@rrz.uni-koeln.de> se@IKP.Uni-Koeln.DE (Stefan Esser) writes:
>The EXABYTE writes 8KByte of data per 'track' and another (from weak
>memory) 4K for ECC and other purposes (eg. logical block sizes).
>
>For each logical block an integral number of 1KB 'sectors' is written, 
>with remainder of the last sector unused, if the the block wasn't a
>multiple of 1KB.
>
>You loose half the capacity, when writing 512 Byte blocks and the per
>command overhead of the 8200 is high, these facts limit the transfer rate 
>to some 10% of its nominal speed.
>
>That got nothing at all to do with inaccuracies of a 'consumer video' 
>product.

Sorry about the "consumer video" crack.

>|> (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.
>
>The drive gathers data in its 240K buffer and starts writing 8K tracks when 
>a certain amount of data is available for immediate transfer.
>There are several parameters which can be adjusted via SCSI mode page
>commands and which allow some tuning of the drive's buffering behaviour.
>You could increase the amount of data required to start transfering data 
>from the buffer to the tape. This might increase throughput and at the 
>same time reduce head wear.

This is an issue for the design engineers.  In this instance, I'm the end
user who had to figure out why the drives kept failing.

>|> (2)  Head wear increases dramatically for the same reason.  Drive failures
>|> will be much more frequent with small block sizes.
>
>Head wear is directly proportional to operation hours, not the amount of 
>data transfered. EXABYTE drives are specified at some 2000 hours, if I'm 
>not too confused.

I didn't mean head wear is directly proportional to the amount of data
transfered.  What I was trying to say was that head wear is proportional to
the number of feet of tape that gets dragged over the drum.

Let me back up a little.  My experience was with a data center with 16
8mm drives doing dumps 6 hours a night.  Failure rate (mostly due to
head wear) was 2 to 3 drives a month averaged over several months.  Hold
that thought for a second.

The inability for Unix to consistently find a file less than 512 bytes was
documented on three different drives: Megatape, Exabyte, and Contemporary
Cybernetics.  An engineer from Contemporary Cybernetics flew out to help us
identify problems.  He's the one who pointed out that we were using the
cpio default block size of 512 bytes which is "not supported by 8mm drives".
When we fixed that, the problem went away on all three vendor's drives.
This is not theory or guesswork, but actual observation.

Shortly after that, the engineering department of one of our 8mm resellers
released a technical bulletin describing the phenomena of increased head wear
on 8mm drives when 512 byte blocks were used.  This is from memory, but I
believe they said that the drive repositions the tape much more often when
small blocks are used, which causes more accumulated feet of tape to come in
contact with the drum for the amount of data written, which effectively
increases tape wear.  Note that we're not talking hours here, we're talking
the number of tapes you can actually write before the drive needs to be
swapped out.

Now, the only empirical data I can use to support the "increased head wear
theory" is that our drive failures dropped to slightly less than one per
month averaged over about 7 months after we fixed the backup program.  What
sullies this observation is that we changed vendors at the same time.  Perhaps
the new drive was simply more reliable.  You decide.

Mind you, this was all on Unix boxes, so perhaps it is a Unix driver issue
after all.  However, when I hear that someone else is having the same problem
we were, I have to think that the first thing to try is a solution that worked
before.


			Ron
-- 

		Barbara Bush - Colin Powell for '92
Newsgroups: comp.periphs.scsi