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

[Marek Marczykowski <marmarek@xxxxxxxxxxxx>]

On 06.09.2011 18:32, 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.

At least in 2.6.38.3 xenlinux  (SUSE). Now I'm not sure if 3.1rc2 also
needed this modification (can't find it now).

>> When I disable barriers (patching blkbackend to set feature-barrier=0)
>> everything works fine with all above versions.
> 
> Ok, and the patch you sent "[PATCH] Initialize vars in blkfront_connect"
> as well?

Yes.
I've noticed now that this patch was needed only on your testing branch
(not vanilla kernel).

>> My setup is xen-4.1.1 (if it matters), backends: phy from device-mapper
>> device and phy from loop device; frontends covered by device-mapper
>> snapshot, which is set up in domU initramfs.
>>
>> It looks like some race condition, because when I setup device-mapper in
>> domU and mount it manually (which cause some delays between steps), it
>> works fine...
>>
>> Have you idea why it happens? What additional data can I provide debug it?
>>
>> In addition it should be possible to disable barrier without patching
>> module... Perhaps some pciback module parameter? Or leave feature-*
> 
> Not sure why you would touch pciback.. 

I mean blkback of course.

> But the barrier should _not_
> be enabled in those backends. The 'feature-flush-cache' should be.

(on 3.1rc2) Looking to xenstore now there is 'feature-flush-cache=1' and
no 'feature-barrier'. So it is ok.

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

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