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From: ron%brl-vgr@sri-unix.UUCP
Newsgroups: net.unix
Subject: Re:  Raw vs. block device. I'm confused.
Message-ID: <15194@sri-arpa.UUCP>
Date: Thu, 12-Jan-84 00:18:07 EST
Article-I.D.: sri-arpa.15194
Posted: Thu Jan 12 00:18:07 1984
Date-Received: Mon, 9-Jan-84 00:13:17 EST
Lines: 14

From:      Ron Natalie <ron@brl-vgr>

In one word..."buffering".  A io on a block device is always done into
a buffer in the kernel.  A raw disk io is done directly into the buffer
the user passed.  Of course raw is faster if you are looking at a block
and then throwing it away.  You don't need the buffer cache and you don't
need all that copying.  There are constraints however.  Since the peripheral
really transfers directly into the user buffer the number of characters
the user requested it may not work.  Most peripherals require word align
ments, various offsets and minimum granularities.  ADB is probably going
away because you are not reading the beginning of a physical disk block,
or you are not reading a whole block.

-ROn