Re: [Xen-devel] [PATCH] xen-blk(front|back): Handle large physical sector disks

[James Harper <james.harper@xxxxxxxxxxxxxxxx>]

> 
> >>> On 15.05.13 at 11:26, Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> wrote:
> > On 14.05.2013 10:04, Jan Beulich wrote:
> >>>>> On 13.05.13 at 19:47, Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> wrote:
> >>> --- a/drivers/block/xen-blkback/xenbus.c
> >>> +++ b/drivers/block/xen-blkback/xenbus.c
> >>> @@ -704,6 +704,13 @@ again:
> >>>                            dev->nodename);
> >>>           goto abort;
> >>>   }
> >>> + err = xenbus_printf(xbt, dev->nodename, "physical-sector-size",
> "%u",
> >>> +                     bdev_physical_block_size(be->blkif->vbd.bdev));
> >>> + if (err) {
> >>> +         xenbus_dev_fatal(dev, err, "writing %s/physical-sector-size",
> >>> +                          dev->nodename);
> >>> +         goto abort;
> >>
> >> Failure here should not be fatal (as with any other protocol
> >> extensions).
> >
> > So I suppose that should be xenbus_dev_error and no abort here. Just
> > wondering
> > (and sorry for being thick headed here) why would a failure here be
> > different in
> > severity for an extension or not. Is that not just adding an element to the
> > xenstore object and failure would not be related to this being an
> extension?
> 
> A driver should only bail upon encountering a problem that it can't
> recover from. Failure to write a xenstore node that neither the
> backend nor the frontend really require for their work is certainly
> not among those. Yes, it's only a xenstore write, but it can fail at
> least theoretically (or else there wouldn't be a need for error
> handling here in the first place), and you shouldn't handle such
> failure in undue ways (i.e. failure to write required nodes is fatal,
> but failure to write nodes related to extensions isn't).
> 

What is the recovery though? If the physical block size is unusual (eg not 512) 
and the write has failed, what is the outcome? I suspect that it's going to not 
be what the user expected - partitions could be incorrectly aligned if doing an 
install, etc. If it were my system then in the (vanishingly rare?) case that 
this write failed, I'd prefer a hard failure.

If a simple write to xenstore fails then isn't the world coming to an end 
anyway?

James


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