Am 21.03.2011 13:23, schrieb Avi Kivity: > On 03/09/2011 07:38 PM, Anthony Liguori wrote: >> On 03/09/2011 11:27 AM, Christoph Hellwig wrote: >>> On Wed, Mar 09, 2011 at 05:15:53PM +0100, Kevin Wolf wrote: >>>> Quoting the bug report: >>>> >>>> qemu ensures that guest writes and qemu metadata writes hit the >>>> disk >>>> when necessary to prevent data corruption. However, if an image >>>> was >>>> in host pagecache prior to starting qemu, for example after >>>> running >>>> qemu-img convert, then nothing prevents writes from reaching the >>>> disk out of order, potentially causing corruption. >>>> >>>> I'm not entirely sure if there is a realistic case where we would get >>>> corruption, but it's probably a case of better safe than sorry. >>> Except for SCSI with ordered tags (which we don't support) there are not >>> ordering guarantees in the storage protocols, and as such the above >>> explanation >>> doesn't make any sense at all. >> >> Even if there was, a guest shouldn't be relying on the ordering of a >> write that comes from a non-guest. >> >> I don't understand the failure scenario here. > > $ cp x.img y.img > $ qemu -drive file=y.img,cache=writeback > <read something from disk, send it over the network> > <no guest flushes> > <host crash> > > The guest may expect that any or none of its writes hit the disk, but > that anything that it read from the disk, stays there.
Is it true for real hardware? Consider a reboot, you could still have some data in a volatile disk write cache if the OS that ran before the reboot hasn't flushed it. Kevin
