🚀 go-pugleaf

In article <242@uwslh.UUCP> dem@uwslh.UUCP (David E. Miran) writes:
>... we are using an SI 9700 tape system on our VAX-11/750 ...
>This is actually a Storage Technology model 1953 Tridensity tape
>drive (800/1600/6250) that runs at 125 ips.  The controller is a single
>board that is both massbus adaptor (or pretends it is) and tape
>controller.  In our experience this unit is reliable and very fast.

But you may not be able to use all that speed.  DEC disk drives
tend to have low peak practical (by which I mean `file system style'
through the raw device, i.e., /etc/dump) transfer rates:  I have
never observed more than 130KB/s from RA81s; I think our RP06s top
out below 200KB/s.  Our Eagles, on an Emulex SC78x, occasionally
send more than 600KB/s, but more usually run in the 200 to 500 KB/s
range.  (Try running systat or iostat while dumping a drive to
/dev/null.)

>It is very expensive (we paid about $30,000 in 1984) for it and it
>is not state of the art (it has been out for at least 5 years).
>On the other hand, it runs at full speed (125ips) even at 6250 bpi
>while many of the modern inexpensive drives are 125ips at 1600 bpi
>and only 50 to 75 ips at 6250 bpi.

At 125 in/s, assuming 32KB blocks (4.3BSD dump), 6250 B/in, .7 in gap:

	32768 B/blk
	----------- ~= 5.25 in (data) + .7 in (gap) = 5.95 in/blk
	 6250 B/in

	5.95 in/blk
	----------- = .0476 s/blk
	 125 in/s

	32768 B/blk
	----------- ~= 688 KB/s
	.0476 s/blk

This means dump must provide at least 688 KB/s to your tape drive,
or your tape is outpacing the rest of your system.  *If* you can
maintain a steady 600KB/s transfer rate from your disks, you can
probably do this.  If you have RA81s, forget it.

(Of course, some clever person may speed dumps somehow, which could
just throw all these calculations out the window.  [Interleaved
file systems, anyone?]  But *at this time* those expensive 125 or
200 ips 6250 bpi drives are not worth it.  We have one, and it *is*
nice, but....)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	seismo!mimsy!chris