Date: Wed, 10 Nov 1999 14:22:58 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Simon Shapiro <shimon@simon-shapiro.org> Cc: freebsd-arch@freebsd.org Subject: Re: I/O Evaluation Questions (Long but interesting!) Message-ID: <Pine.BSF.4.10.9911101412490.12295-100000@alphplex.bde.org> In-Reply-To: <3828BB53.DD482CD2@simon-shapiro.org>
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> And this, ladies and gentlemen is what I do not understand; > > Why is random WRITE to a block device about 10-11 times > slower than raw device? > Actually, sequential read is 1/3 of raw device too. Why? Block devices have to use a fixed block size. This size is normally BLKDEV_IOSIZE. For historical reasons, BLKDEV_IOSIZE is normally too small. On i386's, it is 2048 in RELENG_3 and 4096 in -current. -current has a sysctl to set the default size. Large i/o's are split up into blocks of the fixed size. Small blocks are very bad for sequential i/o's. They may actually be good for random i/o's if the original i/o's are small. Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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