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

[Marek Marczykowski <marmarek@xxxxxxxxxxxx>]

On 06.09.2011 18:55, Konrad Rzeszutek Wilk wrote:
> 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.

2.6.38.3 OpenSUSE (stable branch) uses feature-barrier and no
feature-flush-cache, so it should works...
http://kernel.opensuse.org/cgit/kernel/tree/drivers/xen/blkback/xenbus.c?h=stable#n208
http://kernel.opensuse.org/cgit/kernel/tree/drivers/xen/blkback/xenbus.c?h=stable#n443

> 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).

Ok, it looks like I've mixed up logs from 2.6.38.3 dom0 and 3.1-rc2
dom0. Sorry for that.

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

Yes.

-- 
Pozdrawiam / Best Regards,
Marek Marczykowski         | RLU #390519
marmarek at mimuw edu pl   | xmpp:marmarek at staszic waw pl

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