Re: [Xen-devel] blkfront problem in pvops kernel when barriers enabled

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

On Tue, Sep 06, 2011 at 12:32:13PM -0400, Konrad Rzeszutek Wilk wrote:
> On Sun, Sep 04, 2011 at 12:49:42PM +0200, Marek Marczykowski wrote:
> > Hello,
> > 
> > Pvops block frontend (tested vanilla 3.0.3, 3.1rc2, Konrad's testing
> > branch) produces a lot of I/O errors when barriers are enabled but
> > cannot be used.
> > 
> > On xenlinux I've got message:
> > [   15.036921] blkfront: xvdb: empty write barrier op failed
> > [   15.036936] blkfront: xvdb: barriers disabled
> > 
> > and after that, everything works fine. On pvops - I/O errors.
> > As backend I've used 2.6.38.3 xenlinux (based on SUSE package) and
> > 3.1rc2 with same result.
> 
> Hm, and the 'feature-barrier' was enabled on in those backends?
> That is really bizzare considering that those backends don't actually
> support WRITE_BARRIER anymore.

To be exact:
http://lwn.net/Articles/399715/ so in 2.6.37-era ish the WRITE_BARRIER
functionality got ripped out.

And the LFS summit in 2010 had more details:
http://lwn.net/Articles/399148/
"That led, eventually, to one of the clearest decisions in the first
day of the summit: barriers, as such, will be no more."

And WRITE_BARRIER != WRITE_FLUSH so if the SuSE backend is using it
as so - then there is a bug in there.

In the 3.1-rc2 upstream kernel there should be absolutly no hint
of 'feature-barrier' in the _backend_ code (it is OK for it to be
in the frontend code).

Can you confirm where you got your sources?

P.S.
There should be a backwards compatible way of implementing the
'feature-barrier' in the block backend of 3.0 and further kernels..
but nobody has stepped up in implementing it.

Also, one more thing - are you sure you are using the block backend?
You might be using the QEMU qdisk?

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