8mm tape drives for Pyramid

[Charles Sandel]

We have 2 of the Megatape 8mm units.  In general, they
are good units.  We had previously bought 2 Delta Microsystems
8mm units for use on our Suns and the Megatapes were bought
to be compatible with them.  We had some difficulty
getting that to work, but once the bugs were worked out and
I understood what was going on, the compatibility issue
has been resolved.

The Megatapes use the Exabyte drive and add a Pertec interface
so that they can "look" like  a 1/2" tape drive to the Pyramid.
There is no particular speed or reliability advantage to this.
The advantage is that it does not require any special software
to use.  The usual Unix utitilies for magtape manipulation
(mt, tar, dump, restore, cpio, dd) work fine.  I have found
no problems with the magtape emulation of the Megatape.

Setting up the Megatape requires you to select the "speed"
of its connection to the host computer.  This is DIP-switch
selectable up to 500KB/sec.   For the Pyramid, I found by
experimentation that 250KB/sec is optimum.  In fact, any higher
selection will not work.  This is OK, since the drive itself
has a upper limit of about 200-250KB/sec transfer anyway.
(NOTE: we have a 9825 with the "fast" IOP's.  If you have IOC's
I suspect you might have to select a lower transfer rate.)

There are (as far as I know) 3 recording formats for
8mm tape:
	1. Fixed-block size: blocks are written to tape
	in whatever size you specify.  Each block is assumed
	to be the same size
	2. Variable record size: blocks are written to tape
	including a header which specifies the size of the block
	3. "Short" EOF markers: uses short EOF markers; in other
	modes, an EOF marker uses up 2MB of tape.  In short mode,
	it uses only 1/2 MB, but "skip-file" commands do not
	work.

A tape written in one format (as far as I can tell) is not
readable using a different format.  The Delta Microsystems
boxes will read and write in all 3 formats.  The Megatape
(as far as I can tell) *always* uses the variable-record-size
format.  For compatibility, we are always using our Delta
boxes in the variable-size format.  There does not seem to be any
speed penalty associated with this format over the fixed-size
format.  We are able to exchange tapes between the Delta and
Megatape with no problems, assuming we use the same block
size.  For dumps, using the 4.[23]BSD dump(8) utility, we
always use a 32KB block size on all our dumps to both the
Megatape and Delta.  This is the maximum for the Megatape
and always using the same REAL block size assists
tape transfer between the different types of drives.

NB: the Pyramid dump(8) takes its block specification
in 2KB chunks, while the Sun dump(8) takes it's specification
in 1KB chunks.  Therefore, to get a 32KB blocksize on the
Pyramid we specify a block size of 16 to the Pyramid dump(8)
program, while the Sun gets a specification of 32 for the
same REAL block size.

We have had a couple of problems with the Megatape:
	1. Setting up is difficult because of the position
	of the rear cable sockets
	2. Setting the DIP switches is difficult because
	of their placement
	3. Deciding on the setting for the transmission speed
	was by trial-and-error because there was no clear
	indication of how to determine this (use 250KB/sec).
	4. Tech support is not particularly helpful by comparison
	with other companies (such as Delta)
	5. One drive recently died with a tape stuck inside.
	We had to use the manual door release to get the tape
	out.  Megatape's warranty replacement policy does not
	include provision for a loaner, so we will be without
	a drive for about 3 weeks while they fix it.

In general, though, I am glad we have the drives.  They provide
us with the tape capacity we need on our Pyramid.  If you do
not have Pyramid's SCSI interface (a relatively new product I
think), then you will need to look at Megatape and other companies
that provide a Pertec emulation with the Exabyte drive.
I believe that there is one other company providing the
Pertec interface, but I do not know its name.

Megatape is not the most responsive company I've worked with
as far as technical support (no one seemed to be willing to
answer questions for me, and the warranty repair policy is
not particularly good), but they are far from the worst.

NB: the transfer rate of the Megatape drives on the Pyramid
realistically maxes out at about 180KB/sec for backups.
This is not particularly fast.  With any of the 8mm drives,
you must be willing to trade speed for capacity.

Charles Sandel
MCC/STP
Austin, Texas
sandel at mcc.com
sandel at milano.uucp