Re: qemu write cacheing and DMA IDE writes

["Daniel P. Berrange" <berrange@redhat.com>]

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 <riel@redhat.com>
>   Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>
> 
> 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.
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