Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk

["andrzej zaborowski" <balrog@zabor.org>]

On 31/07/06, Jens Axboe <qemu@kernel.dk> wrote:
> On Mon, Jul 31 2006, andrzej zaborowski wrote:
> > On 30/07/06, Jamie Lokier <jamie@shareable.org> wrote:
> > >Rik van Riel wrote:
> > >> This may look like hair splitting, but so far I've lost a
> > >> (test) postgresql database to this 3 times already.  Not getting
> > >> the guest application's data to disk when the application calls
> > >> fsync is a recipe for disaster.
> > >
> > >Exactly the same thing happens with real IDE disks if IDE write
> > >caching (on the drive itself) is enabled, which it is by default.  It
> > >is rarer, but it happens.
> >
> > The little difference with QEMU is that there are two caches above it:
> > the host OS'es software cache and the IDE hardware cache. When a guest
> > OS flushes its own software cache its precious data goes to the host's
> > software cache while the guest thinks it's already the IDE cache. This
> > is ofcourse of less importance because data in both caches (hard- and
> > software) is lost when the power is cut off.
>
> But the drive cache does not let the dirty data linger for as long as
> wht OS page/buffer cache.

I would say this an argument speaking for actually using O_SYNC.

>
> > IMHO what really makes IO unreliable in QEMU is that IO errors on the
> > host are not reported to the guest by the IDE emulation and there's an
> > exact place in hw/ide.c where they are arrogantly ignored.
>
> Send a patch, I'm pretty sure nobody would disagree :-)

Here's what I proposed:
http://lists.gnu.org/archive/html/qemu-devel/2005-12/msg00275.html but
I'm afraid it's not correct :P

>
> --
> Jens Axboe
>
>


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