Mystical Parameters for 8mm Drive: The Answers

[Mike Bell]

First of all thanks to all the respondents who sent in their drive
parameters: there was a considerable diversity in the responses. Many
proposed parameters were "mystical" - in the sense that they had been
derived by considerable experimentation.

One conclusion was clear, however: whatever you do, don't try and use the
*real* parameters for tape length or density. (My suspicion here is that
the real parameters of length = 346ft, density=4137733 bpi cause an
unchecked integer overflow in dump - since 346 * 12 * 4137733 > 2**32-1)

The clearest answer received (reproduced below) was from
John Gibbins (johng at uniwa.uwa.oz.au):

>>Could anyone recommend the best parameters for using tar or dump with an
>>Exabyte 8200 8mm tape drive? Also, should the tape drive be /dev/rst0 or
>>/dev/rmt0?

Our local Sun office gave me a photocopy of some pages of what appears to
be "EXAbyte EXB-8200 installation guide  Document Rev D (0590)".  The
pages are headed "Dawn Technologies Pty Ltd", and refer to SunOS 4.0.3 and
4.1.  They include the following:

	The EXB uses an internal data block size of 1024 bytes.  For
	correct operation you should use data blocking factors that are
	multiples of this figure.  SunOS 4.0 and later release support
	variable length record sizes, but still efficient use is best
	achieved using block sizes that are multiples of 1024 bytes.
	Depending on the amount of installed memory and disk on your sun
	system, you may choose to use blocking factors of 126, (this
	gives64kb blocks and is very commonly used) 1000 or any even
	number of disk blocks larger than 2 (1 disk block = 512 bytes or
	1/2 an EXB-8200 tape block).
	...
	tar cvfb /dev/rst9 1000 /export/exec <CR>
	...
	and
	...
	For the purposes of the dump and restore commands, assume your EXAtape
	writes at a density of 54,000 bytes per inch and the length of the tape
	is 6,000 feet...
	...
	/etc/dump 0ucbdsf 1000 54000 6000 /dev/rst9 /dev/sd0f <CR>

It did not say which size tape was being used.

The concensus was that the st driver should be used (although note that
the only mention of 8mm drives in the man pages is in the mtio (mt) driver
description - which is why I asked). I've no idea if it makes any
difference.

IMPORTANT ADDITIONAL NOTES 

Note also that dump's initial estimates of time-to-complete can be way
off, and that using a block size of 1000 cripples the ethernet for a
remote dump. (The ioctl call which obtains drive information gets a
recommended block size of 126). tar and restore will need to be told the
block size for very large block sizes, whereas they can figure it out for
a block size of 126 - so mark your dump tapes accordingly. 

My thanks once again to:
 "Ric Anderson" <ric at cs.arizona.edu>
 dnb at meshugge.media.mit.edu (David N.??? Header malformed)
 pierre at csis.dit.csiro.au (Peter Nikitser)
 trn at warper.jhuapl.edu (Tony Nardo)
 gdrew at cs.UMD.EDU (Greg Drew)
 jsalmi at Solbourne.COM (John Salmi)
 mostardi at ux5.lbl.gov (David Mostardi)
 daniel at nstn.ns.ca (Daniel MacKay)
 johng at uniwa.uwa.oz.au (John Gibbins)

(and anyone else whose reply is still in transit...)