Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!mips!apple!sun-barr!newstop!sun!snafu!lm From: lm@snafu.Sun.COM (Larry McVoy) Newsgroups: comp.unix.wizards Subject: Re: Raw disk I/O Message-ID: <131943@sun.Eng.Sun.COM> Date: 16 Feb 90 07:05:24 GMT References: <1990Feb15.212708.19046@ladc.bull.com> Sender: news@sun.Eng.Sun.COM Reply-To: lm@sun.UUCP (Larry McVoy) Organization: Sun Microsystems, Mountain View Lines: 23 In article <1990Feb15.212708.19046@ladc.bull.com> fmayhar@hermes.ladc.bull.com writes: >In article , marc@focsys.uucp (Marc H. >Morin) writes: >> I am investigating using the raw disk interface to increase performance >> of our application. The application is an imaging system, thus the I/O >> consists of large data transfers to and from the disk. >We were looking into this as well. After testing, we decided to >continue to use >the block device, since it was at least three times faster than the raw device. >I concluded then that the block device was doing some disk access optimizations >that the raw device wasn't doing. Also, using the block device has the added >advantage that any new optimizations in the driver would automatically be used >by the application. Under SunOS, at least, the raw device doesn't do read ahead. Some applications consider this a feature, others a bug. To prove this, try $ /bin/time dd if=/dev/rsd0a of=/dev/null bs=8k $ /bin/time dd if=/dev/sd0a of=/dev/null bs=8k --- What I say is my opinion. I am not paid to speak for Sun, I'm paid to hack. Besides, I frequently read news when I'm drjhgunghc, err, um, drunk. Larry McVoy, Sun Microsystems (415) 336-7627 ...!sun!lm or lm@sun.com