From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Daniel P. Berrange" Subject: Re: qemu write cacheing and DMA IDE writes Date: Mon, 25 Feb 2008 17:00:51 +0000 Message-ID: <20080225170051.GJ2614@redhat.com> References: <18370.61781.669132.772706@mariner.uk.xensource.com> Reply-To: "Daniel P. Berrange" Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <18370.61781.669132.772706@mariner.uk.xensource.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Ian Jackson Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org On Mon, Feb 25, 2008 at 04:48:21PM +0000, Ian Jackson wrote: > I've been doing some merge work between tools/ioemu and qemu > upstream. I came across this commit: > > changeset: 11209:9bb6c1c1890a07885265bbc59f4dbb660312974e > date: Sun Aug 20 23:59:34 2006 +0100 > files: [...] > description: > > [qemu] hdparm tunable IDE write cache for HVM > > qemu 0.8.2 has a flush callback to the storage backends, so now it is > possible to implement hdparm tunable IDE write cache enable/disable for > guest domains, allowing people to pick speed or data consistency on a > case by case basis. > > As an added benefit, really large LBA48 IOs will now no longer be broken > up into smaller IOs on the host side. > > From: Rik van Riel > Signed-off-by: Christian Limpach > > However there seems to me to a be a bug in it: it does not take effect > for DMA writes, which are handled by a separate set of functions. > Since most guest operating systems will be using (emulated) DMA, it > seems that the result is that we advertise configurable write cacheing > but in fact in most cases always cache. Are you sure it doesn't apply for DMA writes ? AFAICT, the DMA write is done by ide_write_dma_cb() in hw/ide.c. At the point the DMA transfer completes that method does if (!s->write_cache) bdrv_flush(s->bs); Which should flush the data to disk, if the guest has done hdparm -W 0 on their device. Regards, Dan. -- |=- Red Hat, Engineering, Emerging Technologies, Boston. +1 978 392 2496 -=| |=- Perl modules: http://search.cpan.org/~danberr/ -=| |=- Projects: http://freshmeat.net/~danielpb/ -=| |=- GnuPG: 7D3B9505 F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 -=|