From owner-freebsd-hackers Mon May 17 11:36:23 1999 Delivered-To: freebsd-hackers@freebsd.org Received: from apollo.backplane.com (apollo.backplane.com [209.157.86.2]) by hub.freebsd.org (Postfix) with ESMTP id F419F151BF for ; Mon, 17 May 1999 11:36:19 -0700 (PDT) (envelope-from dillon@apollo.backplane.com) Received: (from dillon@localhost) by apollo.backplane.com (8.9.3/8.9.1) id LAA27148; Mon, 17 May 1999 11:36:17 -0700 (PDT) (envelope-from dillon) Date: Mon, 17 May 1999 11:36:17 -0700 (PDT) From: Matthew Dillon Message-Id: <199905171836.LAA27148@apollo.backplane.com> To: Zhihui Zhang Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: cylinder group and special device References: Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG :Can anyone answer the following two questions for me: : :(1) Does a cylinder group in FFS have to begin at a cylinder boundary? No. The only thing that happens if it isn't is that the '# (Cyl. .... )' comment in the disklabel has asterixes in it. Modern hard drives use variable length tracks so most people do not bother trying to align the placement of filesystemes on the disk. The filesystem doesn't care, you shouldn't care, so don't worry about it. :(2) If we read a block via a special device name (/dev/xxx), will the :block be buffered as normal file data and used when we need the block :again? If you use, for example, /dev/da0a, it will be buffered. If you use /dev/rda0a, it will not be buffered. 'r' means raw-unbuffered. Accessing block devices directly is somewhat dangerous. The system will not allow you to open the buffered device if a mounted filesystem is using the partition, but it will allow you to open the raw device even if a filesystem is using the partition. This can be extremely dangerous so care must be taken when accessing block devices. -Matt Matthew Dillon :Thanks for any help. : :-------------------------------------------------- :Zhihui Zhang. Please visit http://www.freebsd.org :-------------------------------------------------- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message