Re: [Xen-devel] bug when using 4K sectors?

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Sep 05, 2012 at 11:56:08PM +0000, James Harper wrote:
> > On Mon, Aug 13, 2012 at 02:12:58PM +0000, James Harper wrote:
> > > I notice this code in drivers/block/xen-blkback/common.h
> > >
> > > #define vbd_sz(_v)      ((_v)->bdev->bd_part ? \
> > >                          (_v)->bdev->bd_part->nr_sects : \
> > >                           get_capacity((_v)->bdev->bd_disk))
> > >
> > > is the value returned by vbd_sz(_v) the number of sectors in the Linux
> > > device (eg size / 4096), or the number of 512 byte sectors? I suspect
> > > the former which is causing block requests beyond 1/8th the size of
> > > the device to fail (assuming 4K sectors are expected to work at all -
> > > I can't quite get my head around how it would be expected to work -
> > > does Linux do the read-modify-write if required?)
> > 
> > I think you need to instrument it to be sure.. But more interesting, do you
> > actually have a disk that exposes a 4KB hardware and logical sector? So far
> > I've only found SSDs that expose a 512kB logical sector but also expose the
> > 4KB hardware.
> > 
> > Never could figure out how that is all suppose to work as the blkback is 
> > filled
> > with << 9 on a bunch of things.
> > 
> 
> I was using bcache which does expose a 4K block size, by default. I changed 
> it to 512 and it all works now, although I haven't tested if there is any 
> loss of performance.

OK, let me see how I can setup bcache and play with that.
> 
> Does Xen provide a way to tell Windows that the underlying device is 512e (4K 
> sector with 512 byte emulated interface)? This would keep everything working 
> as is but allow windows to align writes to 4K boundaries where possible.

We can certainly expose that via the XenBus interface.
> 
> James

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