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 12:13 +0100, Stefano Stabellini wrote:
> On Tue, 14 Aug 2012, Stefano Stabellini wrote:
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> 
> ping

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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