Re: [Xen-devel] [PATCH] libxl/qemu-xen: use cache=writeback for IDE and cache=none for SCSI

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-08-22 at 14:27 +0100, Ian Campbell wrote:
> On Wed, 2012-08-22 at 12:13 +0100, Stefano Stabellini wrote:
> > On Tue, 14 Aug 2012, Stefano Stabellini wrote:
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > 
> > ping

pong?

> 
> Is this an appropriate change during rcs, is it critical for 4.2 or
> should it wait for 4.3?
> 
> I think the changelog here is rather lacking, which makes it hard for me
> to decide what to do. e.g. the usual stuff: Why are you making this
> change? What is the impact? etc
> 
> What is the default for all these cases?
> 
> > > diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
> > > index 0c0084f..1c94e80 100644
> > > --- a/tools/libxl/libxl_dm.c
> > > +++ b/tools/libxl/libxl_dm.c
> > > @@ -549,10 +549,10 @@ static char ** 
> > > libxl__build_device_model_args_new(libxl__gc *gc,
> > >              if (disks[i].is_cdrom) {
> > >                  if (disks[i].format == LIBXL_DISK_FORMAT_EMPTY)
> > >                      drive = libxl__sprintf
> > > -                        (gc, "if=ide,index=%d,media=cdrom", disk);
> > > +                        (gc, 
> > > "if=ide,index=%d,media=cdrom,cache=writeback", disk);
> 
> Why does the cacheability matter for an empty device?
> 
> > >                  else
> > >                      drive = libxl__sprintf
> > > -                        (gc, 
> > > "file=%s,if=ide,index=%d,media=cdrom,format=%s",
> > > +                        (gc, 
> > > "file=%s,if=ide,index=%d,media=cdrom,format=%s,cache=writeback",
> 
> Does writeback mean anything for a r/o device?
> 
> > >                           disks[i].pdev_path, disk, format);
> > >              } else {
> > >                  if (disks[i].format == LIBXL_DISK_FORMAT_EMPTY) {
> > > @@ -575,11 +575,11 @@ static char ** 
> > > libxl__build_device_model_args_new(libxl__gc *gc,
> > >                   */
> > >                  if (strncmp(disks[i].vdev, "sd", 2) == 0)
> > >                      drive = libxl__sprintf
> > > -                        (gc, "file=%s,if=scsi,bus=0,unit=%d,format=%s",
> > > +                        (gc, 
> > > "file=%s,if=scsi,bus=0,unit=%d,format=%s,cache=none",
> > >                           disks[i].pdev_path, disk, format);
> > >                  else if (disk < 4)
> > >                      drive = libxl__sprintf
> > > -                        (gc, 
> > > "file=%s,if=ide,index=%d,media=disk,format=%s",
> > > +                        (gc, 
> > > "file=%s,if=ide,index=%d,media=disk,format=%s,cache=writeback",
> 
> Why is an SCSI disk treated differently to an IDE one wrt caching?
> 
> Ian.
> 
> 
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