From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [patch] RFC directio: partial writes support Date: Tue, 2 Mar 2010 20:25:02 +1100 Message-ID: <20100302092502.GD8653@laptop> References: <87iq9lxz3t.fsf@openvz.org> <20100301152149.7ce78e14.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dmitry Monakhov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Andrew Morton Return-path: Content-Disposition: inline In-Reply-To: <20100301152149.7ce78e14.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Mon, Mar 01, 2010 at 03:21:49PM -0800, Andrew Morton wrote: > On Thu, 25 Feb 2010 15:45:58 +0300 > Dmitry Monakhov wrote: > > > Can someone please describe me why directio deny partial writes. > > For example if someone try to write 100Mb but file system has less > > data it return ENOSPC in the middle of block allocation. > > All allocated blocks will be truncated (it may be 100Mb -4k) end > > ENOSPC will be returned. As far as i remember direct_io always act > > like this, but i never asked why? > > Why do we have to give up all the progress we made? > > In fact partial writes are possible in case of holes, when we > > fall back to buffered write. XFS implemented partial writes. > > The problem with direct-io writes is that the writes don't necessarily > complete in file-offset-ascending order. So if we've issued 50 write > BIOs and then hit an EIO on a BIO then we could have a hunk of > unwritten data with newly-writted data either side of it. If we get a > bunch of discontiguous EIO BIOs coming in then the problem gets even > messier - we have a span of disk which has a random mix of > correctly-written and not-correctly-written runs of sectors. What do > we do with that? Hmm, what if we're filling in a hole with direct IO? I don't see where blocks allocated in DIO code will be trimmed on a failed write (because it's within isize). This could cause uninitalized data of the block to leak couldn't it?