Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S263730AbUDGQkX (ORCPT ); Wed, 7 Apr 2004 12:40:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S263734AbUDGQkX (ORCPT ); Wed, 7 Apr 2004 12:40:23 -0400 Received: from fw.osdl.org ([65.172.181.6]:3561 "EHLO mail.osdl.org") by vger.kernel.org with ESMTP id S263730AbUDGQkU (ORCPT ); Wed, 7 Apr 2004 12:40:20 -0400 Date: Wed, 7 Apr 2004 09:42:22 -0700 From: Andrew Morton To: Bruce Allen Cc: adi@hexapodia.org, bug-coreutils@gnu.org, linux-kernel@vger.kernel.org Subject: Re: dd PATCH: add conv=direct Message-Id: <20040407094222.3362e5c8.akpm@osdl.org> In-Reply-To: References: <20040406173326.0fbb9d7a.akpm@osdl.org> X-Mailer: Sylpheed version 0.9.7 (GTK+ 1.2.10; i586-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1768 Lines: 35 Bruce Allen wrote: > > > > On modern Linux, apparently the correct way to bypass the buffer cache > > > when writing to a block device is to open the block device with > > > O_DIRECT. This enables, for example, the user to more easily force a > > > reallocation of a single sector of an IDE disk with a media error > > > (without overwriting anything but the 1k "sector pair" containing the > > > error). dd(1) is convenient for this purpose, but is lacking a method > > > to force O_DIRECT. The enclosed patch adds a "conv=direct" flag to > > > enable this usage. > > > > This would be rather nice to have. You'll need to ensure that the data > > is page-aligned in memory. > > > > While you're there, please add an fsync-before-closing option. > > Andrew, am I right that this is NOT needed for the proposed O_DIRECT > option, since open(2) says: > "The I/O is synchronous, i.e., at the completion of the read(2) or > write(2) system call, data is guaranteed to have been transferred." > so the write will block until data is physically on the disk. A conv=fsync option is an irrelevant wishlist item. If you were to provide conv=fsync then dd should still fsync the file after performing the direct-io read or write. You're correct that an fsync after a direct-io read or write should not need to perform any file data I/O. But it will often need to perform file metadata I/O - file allocation tables, inode, etc. direct-io only syncs the file data, not the file metadata. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/