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

[Anthony Liguori]

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.

For instance, the kernel loading code passes a buffer on the stack to 
bdrv_read().

That's a problem with the current patch honestly, we should not rely on 
callers to align buffers.  bdrv_read()/bdrv_aio_read()/bdrv_pread() all 
should be tolerant of unaligned buffers if we're going to support O_DIRECT.

Regards,

Anthony Liguori

> Regards,
>
> Anthony Liguori
>
> > Paul
> >
> >
> >