Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932503AbVJDOdd (ORCPT ); Tue, 4 Oct 2005 10:33:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932504AbVJDOdd (ORCPT ); Tue, 4 Oct 2005 10:33:33 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:28291 "EHLO pentafluge.infradead.org") by vger.kernel.org with ESMTP id S932503AbVJDOdd (ORCPT ); Tue, 4 Oct 2005 10:33:33 -0400 Subject: Re: Using DMA in read/write, setting block size for I/O From: Arjan van de Ven To: Karthik Sarangan Cc: linux-kernel@vger.kernel.org In-Reply-To: <434288E9.3090108@cdac.in> References: <434288E9.3090108@cdac.in> Content-Type: text/plain Date: Tue, 04 Oct 2005 16:33:20 +0200 Message-Id: <1128436401.2922.11.camel@laptopd505.fenrus.org> Mime-Version: 1.0 X-Mailer: Evolution 2.2.3 (2.2.3-2.fc4) Content-Transfer-Encoding: 7bit X-Spam-Score: 2.9 (++) X-Spam-Report: SpamAssassin version 3.0.4 on pentafluge.infradead.org summary: Content analysis details: (2.9 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- 0.1 RCVD_IN_SORBS_DUL RBL: SORBS: sent directly from dynamic IP address [80.57.133.107 listed in dnsbl.sorbs.net] 2.8 RCVD_IN_DSBL RBL: Received via a relay in list.dsbl.org [] X-SRS-Rewrite: SMTP reverse-path rewritten from by pentafluge.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 961 Lines: 23 > Will the underlying block device read a single 256KB block from the hdd > into pcBuffer > or will it read 256KB as a set of smaller blocks? that depends, not all pieces of hardware can do transfers that big (or at least advertise to the kernel that they can); if that's the case the kernel will chop it up. Note:t he kernel will not *guarantee* that it'll be one io either way. So don't depend on it for correctness! Yet of course the kernel will try to optimize as good as possible > Since the buffer is memory aligned will it enable DMA? doesn't matter; DMA will always happen if the HW is capable of it. Since you use O_DIRECT there will also not be a copy (which would happen if you didn't use O_DIRECT) - 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/