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

[Jens Axboe]

On Mon, Jul 31 2006, Jonas Maebe wrote:
> 
> On 31 jul 2006, at 09:08, Jens Axboe wrote:
> 
> >>Applications running on the host can count on fsync doing the
> >>right thing, meaning that if they call fsync, the data *will*
> >>have made it to disk.  Applications running inside a guest have
> >>no guarantees that their data is actually going to make it
> >>anywhere when fsync returns...
> >
> >Then the guest OS is broken.
> 
> The problem is that supposedly many OS'es are broken in this way. See
> http://lists.apple.com/archives/darwin-dev/2005/Feb/msg00072.html

Well, as others have written here as well, then their OS are broken on
"real" hardware as well.

I wouldn't be adverse to a QEMU work-around, but O_SYNC is clearly not a
viable alternative! We could make QEMU behave more like a real hard
drive when it has aio support, "flushing" dirty cache out in a manner
more closely mimicking what a drive would do instead of relying on the
page cache writeout deciding to write it out.

-- 
Jens Axboe