Pertec Tape Drive Interface Musings

[jwsmobile]

On 8/3/2015 9:54 PM, Eric Smith wrote:
> On Wed, Jun 10, 2015 at 9:40 AM, Dennis Boone <drb at msu.edu> wrote:
>> The main limitation seems to be that it's hard to get the (broken) data
>> from a block that had a read error when using SCSI hardware.  There's
>> probably a way around this if one digs into lower layers of SCSI magic;
>> I haven't gone looking.
> Nothing using standard SCSI commands, unfortunately. Might be some
> vendor-unique stuff.
>
>
I was on the SCSI committee when the tape commands were proposed. The 
original that was proposed was to only have a commands which would be on 
disk controllers (who were the main players) to perform backups of disk 
units and restores.  Luckily that effort was headed off by having 
several parties who made tape devices other than Archive join the committee.

Eventually a proposal was made for the capability to recover data when 
blocks on tape exceeded the size of the read request.

I had to explain such things as read reverse, and how EOT processing was 
done on half inch as they all had defined things which made using such 
devices impossible.  The defined command set was still not useful as 
pointed out by Eric in the area of errors.

That tape marks were not records was also a huge debate, and as Chuck 
points out elsewhere it was not handled consistently either.

I still am not happy that the quarter inch products turned out to be 
garbage and noone called them on it.  Even 3m at the time eventually 
admitted that the physical design was flawed for the speeds that most 
drives ran the tape at, but never made clear to people how bad the 
design was.  Only when you tried to restore data and it was crap did you 
become a member of the club that didn't use the QIC devices.

Amazingly I've never had a 4mm or 8mm tape fail to read for media 
reasons.  I'm going on having media from both that are as old as the 
technology.  My 8mm backups have only one bad tape in the pile, and it 
was marked as written "incomplete" and bad at the time of creation.  
Half inch I've had the same problems documented with the quality of the 
media, but it is much older.

thanks
Jim