HP-UX tapes: summary
Bob Niland
hplabs!rjn at hpfcdc.hp.com
Fri Jun 30 13:36:05 AEST 1989
re: > The bottom line is: one cannot convince a Sun to read a HP-UX tape.
True. About once a year, an unhappy customer asks:
"Can I use cartridges to move data between my HP and my XYZZY systems?"
...and less frequently...
"Why did my XYZZY system ruin my HP tape!?".
The answers are...
- no, you can't, and
- yes, that can happen
There are a number of formats employing seemingly identical 1/4-inch
cartridges. The most common are the various QIC definitions (QIC =
Quarter Inch Committee). Sun and Apollo systems, for example, often use
QIC-24 (60 Mb). Traditional HP drives do not use the QIC format, but
rather one invented by 3M some five years before QIC. It is called HCD
format.
Inserting an HP cartridge in a QIC drive leads at the very least to
frustration, and potentially to permanent damage to the HP tape.
--------------------------------------
The one-minute summary
Attempting to use a QIC tape in an HP HCD drive results in:
0. Tape rejected.
1. Possible tape unspool after repeated attempts.
Attempting to use an HCD tape in a QIC drive results in:
1. No data exchanged.
2. An apparently damaged tape (rewind scenario).
3. A probably destroyed tape if a manual (reposition spill scenario).
4. A definitely destroyed tape (write scenario).
--------------------------------------
Cartridge Tape Specification Overview:
.====================.=========================.==========================.
| Characteristic | HCD Cartridge Tape | QIC Cartridge Tape |
|====================|=========================|==========================|
| Approx. Incep. date| 1980 | 1985 |
|--------------------|-------------------------|--------------------------|
| Mechanical form | 3M DC600 | 3M DC600 |
| factor | | |
| Merchandising name | <none, really> | QIC-24 or QIC-120 |
|--------------------|-------------------------|--------------------------|
| Typical supplier | HP (for HP customers) | any discounter |
|--------------------|-------------------------|--------------------------|
| Generic designation| DC600HC, DC615HC, | DC600A, DC615A |
| | DC600XTD, DC615XTD | |
|--------------------|-------------------------|--------------------------|
| Tape pre-format | Full-track 3M HCD-75 | <none> |
| | or modified HCD-134 | |
| Data format | MFM | NRZI |
|--------------------|-------------------------|--------------------------|
| EOT/BOT | Delimited by pre-format | Optical tape sense holes |
| Rewind position | Right spool empty. | Left spool empty. |
|--------------------|-------------------------|--------------------------|
| Capacity | 67 or 134 Mbytes | 60 or 120 Mbytes |
| Number of tracks | 16 or 32 | 9 or 15 |
|--------------------|-------------------------|--------------------------|
| Access types | Random (re-writeable) | Serial only |
| | or serial | |
| Average seek (60M) | 2 minutes | 20 minutes |
|--------------------|-------------------------|--------------------------|
| Error control | Read-while-write | Read-while-write only |
| | plus ECC | |
`========================================================================='
As you can see, HCD has some advantages. Errors missed by
read-after-write may still be corrected by ECC when read later. Tapes may
be "certified" and re-certified by end users, sparing bad blocks. The
random-access capability allows software to treat the tape like a *very*
slow disk drive. Disk-image cartridge tape backups are mountable as
read/write volumes, and may be safely written upon. Random access is
faster, because the location of every record can be calculated, due to the
pre-formatting. An exhaustive search is not required. This decade-old
format has given us satisfactory service. But, enough drifting away from
the topic...
HCD and QIC cartridges are mechanically identical. An HCD cartridge will
fit in a QIC drive, and vice-versa.
So what is the problem?
* HCD tapes are preformatted by 3M or a 3M licensee. A full-track factory
write head lays down fixed physical records on the tape. No HCD drive
ever writes on these record headers (called "keys"); only in between
them. HP "format" and/or "mediainit" user processes merely "certify",
performing read/write tests, sparing bad blocks and updating logs.
The read/write heads in the drives are either 1/16- or 1/32-track. Keys,
being full-track, cannot be re-written in the field.
* The keys also do not extend to the physical BOT/EOT sense holes in the
tape. When the first HP drives were developed in the early 80s, optical
sensing was deemed too unreliable, so all HP drives use boundary keys to
denote BOT/EOT. (QIC tapes are more like traditional 1/2-inch 9-track
media. The tapes have no pre-formatting, so QIC drives must rely on the
sense holes for BOT/EOT.)
* If you put a QIC tape (blank or written) in an HP HCD drive, the HP drive
will search uptape for the nearest key, fail to find one, time-out, buzz,
release (unload) the tape and illuminate the FAULT indicator. No data
lost, but no data is read or written either. Repeated attempts risk a
tape spill.
* If the you put an HCD tape in a QIC drive, the drive will rewind it (to
the wrong end of the tape by HP conventions) and OUTSIDE the keys region.
Suppose the tape is removed from the QIC drive and re-inserted in an HP
drive. The HP drive, as in the preceding example, will search "uptape"
for a key (again, in the wrong direction due to the QIC rewind), not find
a key in a reasonable time, and reject the tape for fear of spilling tape
if the search continues. It will buzz, release and FAULT. Repeated
attempts risk a tape spill.
The tape may be MANUALLY wound, in the COUNTER-INTUITIVE direction, to
reposition it inside the keys. If re-inserted in the HP drive, it will
properly load (after adding an extra minute to the load time because it
was rewound to the wrong end).
I say "counter-intuitive" because tape motion inside the cartridge is in
the OPPOSITE direction from drive wheel motion, and the tensioning band
makes it difficult to assess which way the tape is supposed to spool.
Now, also please note that in the construction of DC600 media, the tape
ends are not affixed to the reels...
* Given a mis-positioned cartridge, should someone manually wind in the
intuitive {i.e. incorrect} direction, or risk repeated access attempts,
the tape unspools. Disassembly and re-spooling of DC600 tapes is a
nightmare for the inexperienced. In a past life I re-loaded several
DC100A cartridge tapes. Not fun. The probability of reloading the tape
without damage is low.
* If, on the other hand, the HCD tape was left in the QIC drive, two more
scenarios obtain. The QIC drive has no clue that this is an HCD tape.
1. The HCD data structures just look like noise. Any attempt to read
an HCD tape simply fails or returns garbage data. If the tape is
removed after a read attempt, the rewind scenario above prevails.
2. A QIC drive will happily WRITE on an HCD tape. This destroys both
existing data AND the HCD preformatting keys. If the preformatting is
ever lost, the tape can NEVER be used in an HCD drive again. (This
problem can also happen subsequent to degaussing an HCD tape.)
What happens back at the HP drive? Apart from the rewind/time-out
problem, as soon as the HP drive detects a trashed key, it will FAULT
and reject the tape. Only 3M can (re)format an HCD tape, and I doubt
that they offer such a service.
These scenarios are not hypothetical. It happens. Let's be careful out
there. If you have a tape labelled DC6xxHC, DC6xxXTD or with the [(hp)]
logo on it, don't insert it in a tape drive not known to be HP-compatible.
Regards, Hewlett-Packard
Bob Niland 3404 East Harmony Road
ARPA: rjn%hpfcrjn at hplabs.HP.COM Fort Collins
UUCP: [hplabs|hpu*!hpfcse]!hpfcla!rjn CO 80525-9599
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