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Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs


From: Alexandru Avadanii
Subject: Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs
Date: Fri, 31 Mar 2017 19:07:06 +0000

Hi,

> -----Original Message-----
> From: Max Reitz [mailto:address@hidden
> Sent: Friday, March 31, 2017 8:57 PM
> To: Ciprian Barbu; address@hidden; Eric Blake; Alexandru Avadanii
> Cc: Jeff Cody; Markus Armbruster; svc-armband; Kevin Wolf
> Subject: Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs
> 
> On 31.03.2017 19:49, Ciprian Barbu wrote:
> > Hi,
> >
> > Thank you for getting back!
> >
> > I'm trying to follow you, but I don't understand all the details. I would 
> > like to
> ask this question though:
> >
> > What is the difference between v2.8.0 and this commit? With v2.8.0 the
> same qemu command worked, but I admit it doesn't request sharing.
> 
> The difference is that we now make sure there are for example not two
> writers for the same block device (unless both agree that it's OK to have
> shared writers). We haven't done that before.
> 
> The built-in NBD server is completely OK with writes by other parties (such as
> guest devices), but guest devices generally are not (because the guest OS
> should know about this).
> 
> Therefore, you'd now have to set the share-rw flag for guest devices which
> should allow shared writers. Otherwise you cannot set up an NBD server on a
> block device that is in use by the guest device.

Thank you for the detailed explanation!

> 
> > We also use libvirt v1.3.4, which might be a problem, but at least we want
> to understand if the commit in question introduced an obvious problem or if
> it's all in the details.
> 
> Well, I don't think there is any libvirt version that knows about this flag 
> yet.
> 
> ...maybe another way to resolve the issue would be to set share-rw to true
> by default for now, noting that this will be changed in the future.
> In any case, I'm afraid we'll have to wait until Kevin is around on Monday.

Ciprian is gone for the week-end too, so next week it is :)
I think we will change the default for now in our builds, and wait for the 
proper support in Libvirt.
Ty for the default suggestion, btw!

BR,
Alex

> 
> > Btw, the qemu command generated by libvirt is this one, sorry about that:
> 
> And sorry about not yet having thanked you for reporting this issue. :-)
> 
> Max
> 
> > 2017-03-31 17:40:10.956+0000: starting up libvirt version: 1.3.4,
> > package: 0+amos3~u16.04 (Enea Armband Devops Team
> <address@hidden>
> > Fri, 13 Jan 2017 02:06:05 +0100), qemu version: 2.8.50(Debian
> > 1:2.9+amos2~u16.04), hostname: node-2.domain.tld LC_ALL=C
> > PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
> > QEMU_AUDIO_DRV=none /usr/bin/kvm -name
> > instance-00000076,debug-threads=on -S -object
> > secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-14-i
> > nstance-00000076/master-key.aes -machine
> > virt-2.8,accel=kvm,usb=off,gic-version=3 -cpu host -m 256 -realtime
> > mlock=off -smp 1,sockets=1,cores=1,threads=1 -uuid
> > 2812f3c9-f564-499b-a8c7-e9e7ccf24143 -no-user-config -nodefaults -
> chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-14-
> instance-00000076/monitor.sock,server,nowait -mon
> chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew -
> no-shutdown -boot strict=on -kernel /var/lib/nova/instances/2812f3c9-f564-
> 499b-a8c7-e9e7ccf24143/kernel -initrd /var/lib/nova/instances/2812f3c9-
> f564-499b-a8c7-e9e7ccf24143/ramdisk -append 'root=/dev/vda1 rw rootwait
> console=tty0 console=ttyS0 console=ttyAMA0' -device i82801b11-
> bridge,id=pci.1,bus=pcie.0,addr=0x1 -device pci-
> bridge,chassis_nr=2,id=pci.2,bus=pci.1,addr=0x0 -usb -drive
> file=/var/lib/nova/instances/2812f3c9-f564-499b-a8c7-
> e9e7ccf24143/disk,format=qcow2,if=none,id=drive-virtio-
> disk0,cache=none,aio=native -device virtio-blk-device,scsi=off,drive=drive-
> virtio-disk0,id=virtio-disk0,bootindex=1 -drive
> file=/var/lib/nova/instances/2812f3c9-f564-499b-a8c7-
> e9e7ccf24143/disk.config,format=raw,if=none,id=drive-virtio-
> disk1,cache=none,aio=native -device virtio-blk-device,scsi=off,drive=drive-
> virtio-disk1,id=virtio-disk1 -netdev tap,fd=27,id=hostnet0 -device virtio-net-
> device,netdev=hostnet0,id=net0,mac=fa:16:3e:82:0a:2b -serial
> file:/var/lib/nova/instances/2812f3c9-f564-499b-a8c7-
> e9e7ccf24143/console.log -serial pty -vnc 0.0.0.0:0 -k en-us -device
> VGA,id=video0,vgamem_mb=16,bus=pci.2,addr=0x1 -device virtio-balloon-
> device,id=balloon0 -msg timestamp=on Domain id=14 is tainted: high-
> privileges
> >
> > Regards,
> > /Ciprian
> >




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