dal_allan at mcimail.com
Tue Jul 9 21:13:19 PDT 1996
* From the SCSI Reflector, posted by:
* "Dal Allan" <dal_allan at mcimail.com>
I've been following the tape thread which is titled Tape on PLDA but seems
to be on scsi at symbios most of the time though it has also turned up on
disk_attach at dt.wdc.com. Rather than subject both reflectors to this email,
this is going only to scsi. BTW, although it is entitled as a PLDA issue
the problems and solutions seem more generic than FC-specific.
First up, am I listening to the wrong people? The tape vendors which have
chatted to me seem to have a lot fewer problems with the shift to serial
than most of the ones on the reflector(s) today.
I've been reading the notes on tape with interest, and so far find it
interesting that tape vendors are not firing missives. Actually, the traffic
looks to have too many assumptions in it about the way tapes can or will
Doug's paradigm provides a model of expectations that the host has of the
tape drive. With that information, the designer can go off and build a
device that meets them. For years now, tape vendors have been masking
physical problems in media movement from the host.
I don't buy the assumption that a really high speed drive will drop itself
back from streaming mode because of the 'fill buffer first' paradigm. The
faster the device the more pressure there is to keep streaming because you
are talking huge database dumps and the like. Tape guys are just as
innovative as disk guys and host guys, they'll figure out how to meet the
paradigm AND keep streaming. It may not be first generation, they'll be
focusing on simple transition, but count on it to come soon thereafter.
Don't take this wrong and assume I don't support the ideas coming out. One
of the biggest problems that tape drives have had is that host drivers have
been inconsistent in what they expect of the drive. What Doug has started on
describes a model which all hosts will adopt if it gets documented in the
profile. Wow, that's a huge step forward, the device guy can count on
consistent behavior and if it doesn't occur can point to the profile as the
correct way to manage the device.
Concentrating on what the host wants to see as behavior and documenting what
is expected of the drive looks like sufficient guidance to me.
Trust your local tape vendor.
- What happens if a vendor calculates that a 500 KB buffer is sufficient to
sustain streaming and tells the host it can handle 1 MB?
In this competitive world, shaving cost is what its all about.
- Are you going to tell me it is non-compliant?
If a failure occurs and the tape drive is repositioned to the point where
it was before that command began execution the host can't tell that the
tape drive lied because the recovery will work correctly.
As long as the focus is on the paradigms of host expectations, tape vendors
can tell us if there are any invalid assumptions or are non-workable.
More information about the T10