From: utzoo!decvax!cca!mo@Lbl-Unix@sri-unix
Newsgroups: net.unix-wizards
Title: Re: I/O Error in Push
Article-I.D.: sri-unix.4250
Posted: Sat Nov 20 01:22:45 1982
Received: Sun Nov 21 02:11:44 1982

From: mo at Lbl-Unix (Mike O'Dell [system])
Date: 16 Nov 1982 13:18:14-PST
There is a tangentially-related bug in the disk drivers
for 4.1 - they don't do partial reads.  If you want to
back-up an RM05 and say

	dd if=/dev/rhp0c of=/dev/rhp2c bs=100b

(do 50Kbyte reads!)
you will discover the last transfer gets dropped as an error.
The size you give bs MUST integrally divide the size of the
disk partition.  This is even more annoying when trying to copy
only one partition - some of them aren't even an integral
number of tracks!

The comparison in the hp disk driver says

	if (base_address_of_transfer + size > MAX_BLOCK_OF_DISK) {
		u.u_error = E_SCREWED;
		return;
	}

I would claim the raw disk should act like raw tape in that you should
in fact get what data it could give you and return the foreshortened
length.  The current behavior causes me to have to use

	dd if=/dev/rhp0c of=/dev/rhp2c bs=32b 

and do track-at-a-time reads.

	-Mike