tape streamers question

[Chris Lewis]

In article <379 at ispi.UUCP> jbayer at ispi.UUCP (Jonathan Bayer) writes:

>BTW, Bell does have a very good streaming mode.  Depending on the
>software they can stream several megs of data all at the same time,
>while SCO can only stream 20 to 40 K at one time.  However, I don't know
>how much of a speed increase it results in.

In the version of BT's drivers I saw (about 10 months ago), their
"streaming" mode consisted of a *user* program that used a large buffer in
a pipeline between tar/cpio/dump and the special device.  The "large"
buffer was 3Mb (!) by default.  

In a word - whoopie....

I tried to convince ISC to write their own drivers instead - *anybody*
could do better than the wt driver was like, but they didn't until very 
recently - the new ones appear to be worse.  Wonderful....

On 386/ix or BT or Microport or AT&T you can use dd with 1-3Mb buffers
for exactly the same effect.  In SCO the max buffer size for dd is only around 
32K, so you'd be best off writing your own:

	#define	MAX	(1024*1024)	/* big...*/
	main() {
	    extern char *malloc();
	    int n;
	    char *buffer = malloc(MAX);
	    while((n = read(0, buffer, MAX)) > 0) write(1, buffer, n);
	}

Error checking is left as an exercise for the reader....

What I would be really impressed with is someone who wrote a tape driver
that actually had a chance to keep up all by itself.

I have written a multi-process multi-buffer shared memory copy program,
that can stream disk images from one end of the tape to the other without 
stopping, but I'll wait to see what Roe's looks like before posting anything.

[I've got a partially working buffered streaming tape driver - I'll let you 
all know whether I impress myself or not...]
-- 
Chris Lewis, Markham, Ontario, Canada
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