From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request Date: Fri, 28 Mar 2008 13:31:16 -0300 Message-ID: <20080328163116.GA18853@dmt> References: <20080328150517.GA18077@dmt> <20080328150703.GA19624@shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel , qemu-devel@nongnu.org, Avi Kivity To: Jamie Lokier Return-path: Content-Disposition: inline In-Reply-To: <20080328150703.GA19624@shareable.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces@lists.sourceforge.net Errors-To: kvm-devel-bounces@lists.sourceforge.net List-Id: kvm.vger.kernel.org On Fri, Mar 28, 2008 at 03:07:03PM +0000, Jamie Lokier wrote: > Marcelo Tosatti wrote: > > Its necessary to guarantee that pending AIO writes have reached stable > > storage when the flush request returns. > > > > Also change fsync() to fdatasync(), since the modification time is not > > critical data. > > + if (aio_fsync(O_DSYNC, &acb->aiocb) < 0) { > > > BDRVRawState *s = bs->opaque; > > - fsync(s->fd); > > + raw_aio_flush(bs); > > + fdatasync(s->fd); > > + > > + /* We rely on the fact that no other AIO will be submitted > > + * in parallel, but this should be fixed by per-device > > + * AIO queues when allowing multiple CPU's to process IO > > + * in QEMU. > > + */ > > + qemu_aio_flush(); > > I'm a bit confused by this. Why do you need aio_fsync(O_DSYNC) _and_ > synchronous fdatasync() calls? Aren't they equivalent? fdatasync() will write and wait for completion of dirty file data present in memory. aio_write() only queues data for submission: The "asynchronous" means that this call returns as soon as the request has been enqueued; the write may or may not have completed when the call returns. One tests for completion using aio_error(3). So fdatasync() is not enough because data written via AIO may not have been reflected as "dirty file data" through write() by the time raw_flush() is called. The SCSI and IDE drivers use flush() in response to a "flush cache" request, which is used by the guest OS to implement barriers, for example. ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace