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Re: [win-pv-devel] xenvbd (8.x) - blkback/tapdisk3 problems


On Mon, 31 Oct 2016, Paul Durrant wrote:

-----Original Message-----
From: Martin Cerveny [mailto:martin@xxxxxxxxx]
Sent: 30 October 2016 12:18
To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
Cc: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Subject: RE: [win-pv-devel] xenvbd (8.x) - blkback/tapdisk3 problems


Thanks, for response.

On Fri, 28 Oct 2016, Paul Durrant wrote:
-----Original Message-----
I have problems with xenvbd (8.x). There was NOT problem with older pv-
drivers xenvbd (7.2x).
Questions @ bottom.

I use remote raw disk as source (multipath+iscsi+iser+ib).
Two configs:


1) use direct blkback (format=raw, vdev=hda, access=rw,

Performance is good, but __unusable__ for working.

Every few seconds/minutes (randomly, depends on disk load) the
hung on io-operations. I usually saw this more often during write

Sometimes (1:10) I saw "PdoReset" in "DebugView" (DomU):

00003034        10:12:32        XENVBD|__PdoReset:Target[0] ====>
00003054        10:12:53        XENVBD|__PdoReset:Target[0] <====

There is also restart log in Dom0, but no errors on disks/iscsi:

[ 3919.034421] xen-blkback:backend/vbd/3/768: prepare for reconnect
[ 3919.039869] xen-blkback:ring-ref 32, event-channel 40, protocol 1

Yes, XENVBD is being asked to reset because Windows thinks the storage
is stalled and it looks like it was probably right. Suggests a loss of
event notification somewhere.

Ok, I suppose Dom0 logs are result of DomU reset, no problem.

- Windows7 x64
- tested signed winpv drivers 8.1 and primary on development drivers 8.2
- xen 4.5.3, 4.6 and primary 4.7.0
- kernels "XenServer" - kernel-3.10.41-353.380450 (and others from XS6.5)
and kernel-3.10.96-495.383045.x86_64 (and others from XS7)
- blktap3 - blktap-3.0.0.xs1001-xs6.5.0 and blktap-3.2.0.xs1087-



What is buggy in "direct blkback" chain ?
No idea. Possibly blkback, possibly the underlying storage. Your kernels are
old and blkback has undergone many changes in more recent kernels.

(I suppose that XS-kernel is super-tuned for Dom0 and there won't be such

Now updated Dom0:
- fedora 24 + update
- kernel - 4.7.9-200.fc24.x86_64
- xen 4.7.0 + some backports from XS

The problem is the same.
When were "changes" to blkback applied ?

Clearly a 4.7 kernel is about as up-to-date as you want and I am not aware of 
any changes in blkback since then so it does tend to suggest the problem is 
elsewhere. It could well be that XENVBD has some bad interaction with 
blkback... In my test environments I normally use QEMU qdisk as a backend. Do 
you see the same issues if you, say, point blkback at a loopback file mount or 
even an nbd device?

Ok. I will prepare and test and debug more in next weeks.

Was it tested ?
By XenServer? No. XenServer makes no use of blkback.
:-((( (I see few patches to blkback (https://github.com/xenserver/linux-
Well I exaggerated... We need it for corner cases (e.g. early HVM guest boot 
where QEMU is emulating h/w), but none of the 'official' XenServer SRs will use 
it once PV drivers are up and running in the guest; the frontends talk straight 
to tapdisk3 in userspace.


Thanks, Martin

Thanks for answers or hints, Martin Cerveny
Thanks for answers, Martin Cerveny

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