Re: [Qemu-devel] buffer alignment for block backends

[Gerd Hoffmann]

On 04/09/09 17:21, Samuel Thibault wrote:
> Gerd Hoffmann, le Thu 09 Apr 2009 15:54:34 +0200, a écrit :
> > On 04/09/09 15:30, Stefano Stabellini wrote:
> > > The userspace\kernelspace distinction does not make much sense in
> > > minios, but yes it provides an interface to communicate with blkback in
> > > dom0. This interface is used by block-vbd in qemu instead of
> > > block-raw-posix when run in minios.
> > Sit back, look again.  Do you really need it being page-aligned?
>
> Page aligned is an optimization which comes for free when you can
> already choose the alignment.

Ok, so you don't have to.

Does it have an effect positive actually?  i.e. do you have requests 
which would fit into one page but actually span two due to the misalignment?

At least when watching the linux kernel access pattern I see larger 
requests being page aligned anyway.  Probably page cache reads and writes.

> > The xen block protocol can handle unaligned requests just fine.
>
> Not completely arbitrarily aligned requests. They need to be
> sector-aligned.

Agreed.  Sector-alignment you have anyway, thats why I didn't wrote that 
explicitly.

cheers,
  Gerd