TC11/Cypher Tape Drive Problem (Software? Hardware?)

[chris at mimsy.UUCP]

In article <908 at cci632.UUCP> djh at cci632.UUCP (Daniel J. Hazekamp) writes:
>... As it turns out, the 4.3bsd uda50 driver increases the DMA Burst
>size from the default of 0 to 3.

The default is actually 1 word.  It is increased to 4 words.  This
raises the peak transfer rate from ~550K/s to ~750K/s.  But anyway...

>This causes it to hog the bus for extended periods, during which the
>TC-11 stretches out the Inter-Record Gap. To fix the problem change
>the line the reads:
>
>	#define UDABURST 4
>
>to read:
>
>	#define UDABURST 2

There is an easier way, though it will not last past the next kernel
recompile:

	% adb -w /vmunix
	udaburst?W 2
	_udaburst:
	_udaburst:	0	=		2
	$q
	% shutdown
	...
	# fastboot

If you have multiple UDA50s---why bother; actual I/O rate per drive
seems to peak around 125K/s, so 4 drives will consume < 500K/s,
which is below the bus rate---but if you have more than one UDA50,
this will change only the first's burst rate.  Add a `udaburst+4?W
2' to set uda1, and a `udaburst+8?W 2' to set uda2, and so forth.

This will, however, slow down your UDA50 accesses.  Perhaps a better
fix is to mark the TC11 as a `bus hog' a la the RK07s.  Alas, the
4.3BSD UDA50 driver ignores the bus locking protocol.  A fix (or
more precisely, a replacement UDA50 driver) is forthcoming.  (Yes,
we finally started installing 4.3 here yesterday.)

(Another alternative is to move the TC11 and the UDA50 to separate
Unibi, of course.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris at mimsy.umd.edu