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From: alan@shodha.dec.com ( Alan's Home for Wayward Notes File.)
Newsgroups: comp.unix.ultrix
Subject: Re: dump on old 1.2 Ultrix...
Summary: Making streaming tape drives stream...
Message-ID: <376@shodha.dec.com>
Date: 31 Jul 89 20:15:59 GMT
References: <412@wuee1.wustl.edu> <1226@gvgpsa.GVG.TEK.COM>
Lines: 35


	In Ultrix V2.0 a feature was added to the drivers
	of many character special devices (disk and tape).
	To quote the man page title (nbuf(4)):

		nbuf - select multiple-buffer operation to a 
			raw device 

	Dump(8), tar(1) and dd(1) were modified to take advantage
	of this feature.  I'm not sure about cpio and ltf.  THis
	is the driver change that was mentioned in a previous
	posting.  By itself it didn't do anything, but made it
	possible for selected utilities to get better performance
	from stream tape drives.  It's also possible to use it
	with disk devices.

	Before this feature was added one very tacky trick I used
	to make a TK50 stream was to write a filter to sit between
	dump(8) and the tape drive.  The filter had a VERY large
	buffer (4+ MB) that it would fill from stdin.  When the 
	buffer was full it would start writting it to stdout (usually
	pointed at the tape drive.  Writting from memory was fast
	enough to make the TK50 stream.  The program didn't deal with
	dumps that would require more than one tape, but with a little
	work (SMOP) it could probably be added.

	Another tape feature added to V2.0 was end-of-tape detection.
	Rather than just get an I/O error and not know what to do
	with it, programs can now find out more information from the
	driver.  If you know the tape is at EOT it's relatively easy
	to ask for another tape.  Dump(8), tar(1) and dd(1) were
	changed to take advantage of this feature.  If you give a
	long tape length to dump it will use all the tape and do the
	right when it gets to end.  If you leave it to it's estimates
	it can be wrong and waste some tape.