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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 windows
hung on io-operations. I usually saw this more often during write operations.

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 (x86_64-

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-xs7.0.0.x86_64



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 ?

Was it tested ?
By XenServer? No. XenServer makes no use of blkback.

:-((( (I see few patches to blkback 

Thanks for answers or hints, Martin Cerveny

Thanks for answers, Martin Cerveny

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