Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT)

[Paul Brook <paul@codesourcery.com>]

On Wednesday 21 May 2008, Anthony Liguori wrote:
> Paul Brook wrote:
> >> When sector-aligned guest offsets are converted to sector-unaligned
> >> writes (e.g. due to qcow2 etc.), that property is no longer satisfied,
> >> and power failure of the host disk can cause more damage than the
> >> guest is designed to be resistant to.
> >
> > Seems like the easiest solution would be to have qcow always align its
> > writes. We don't do on the fly compression, so it should be fairly easy
> > to make this happen with minimal overhead.
>
> That's not sufficient. O_DIRECT imposes not only offset alignment
> requirements but also requirements on the buffer being read to. Most of
> the code in QEMU does not properly align the read/write buffers.

In that case you need both. For correct operation the qcow layer needs to 
ensure that all file offsets are block aligned (amongst other things, I 
wouldn't be surprised if there are more subtle problems with metadata 
updates).

The memory buffer alignment can occur wherever is most convenient, that's 
trivially atomic w.r.t. unexpected interruptions.

Paul