Re: [Xen-devel] [PATCH 1/3] qemu-xen-traditional: use O_DIRECT to open disk images for IDE

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

> -----Original Message-----
> From: Stefano Stabellini [mailto:stefano.stabellini@xxxxxxxxxxxxx]
> Sent: Monday, March 26, 2012 6:21 PM
> To: Zhang, Yang Z
> Cc: Stefano Stabellini; xen-devel@xxxxxxxxxxxxxxxxxxx; Ian Jackson
> Subject: RE: [Xen-devel] [PATCH 1/3] qemu-xen-traditional: use O_DIRECT to
> open disk images for IDE
> 
> On Mon, 26 Mar 2012, Zhang, Yang Z wrote:
> > > -----Original Message-----
> > > From: xen-devel-bounces@xxxxxxxxxxxxx
> > > [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of Stefano Stabellini
> > > Sent: Friday, March 23, 2012 10:51 PM
> > > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > Cc: Ian.Jackson@xxxxxxxxxxxxx; Stefano Stabellini
> > > Subject: [Xen-devel] [PATCH 1/3] qemu-xen-traditional: use O_DIRECT to
> open
> > > disk images for IDE
> > >
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > ---
> > >  xenstore.c |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/xenstore.c b/xenstore.c
> > > index 4c483e2..ac90366 100644
> > > --- a/xenstore.c
> > > +++ b/xenstore.c
> > > @@ -643,7 +643,7 @@ void xenstore_parse_domain_config(int hvm_domid)
> > >       }
> > >              pstrcpy(bs->filename, sizeof(bs->filename), params);
> > >
> > > -            flags = BDRV_O_CACHE_WB; /* snapshot and write-back */
> > > +            flags = BDRV_O_NOCACHE;
> > >              is_readonly = 0;
> > >              if (pasprintf(&buf, "%s/mode", bpath) == -1)
> > >                  continue;
> > Any reason for this change?
> 
> performance and correctness: BDRV_O_NOCACHE in QEMU means opening the
> disk image with O_DIRECT
Doesn't cache mode have better performance than NOCACHE?

best regards
yang

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