From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758282Ab0EUPVO (ORCPT ); Fri, 21 May 2010 11:21:14 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:57689 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753567Ab0EUPVL (ORCPT ); Fri, 21 May 2010 11:21:11 -0400 Date: Fri, 21 May 2010 11:21:11 -0400 From: Christoph Hellwig To: Josef Bacik Cc: linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, hch@infradead.org, akpm@linux-foundation.org Subject: Re: [PATCH 3/6] direct-io: do not merge logically non-contiguous requests Message-ID: <20100521152111.GB16049@infradead.org> References: <1274300694-8976-1-git-send-email-josef@redhat.com> <1274300694-8976-3-git-send-email-josef@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1274300694-8976-3-git-send-email-josef@redhat.com> User-Agent: Mutt/1.5.19 (2009-01-05) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 19, 2010 at 04:24:51PM -0400, Josef Bacik wrote: > Btrfs cannot handle having logically non-contiguous requests submitted. For > example if you have > > Logical: [0-4095][HOLE][8192-12287] > Physical: [0-4095] [4096-8191] > > Normally the DIO code would put these into the same BIO's. The problem is we > need to know exactly what offset is associated with what BIO so we can do our > checksumming and unlocking properly, so putting them in the same BIO doesn't > work. So add another check where we submit the current BIO if the physical > blocks are not contigous OR the logical blocks are not contiguous. This gets us slightly less optimal I/O patters for other filesystems in this case. But it's probably corner case enough to not care and make it the default. But please make the comment in the comment as verbose as the commit message so that people understand why we're doing this when reading the code in a few years.